Significant Increase In Micronucleus Frequency Research Articles

Effect of smoking habit on the frequency of micronuclei in human lymphocytes: results from the Human MicroNucleus project.

The effect of tobacco smoking on the frequency of micronuclei (MN) in human lymphocytes has been the object of many population studies. In most reports, the results were unexpectedly negative, and in many instances smokers had lower frequencies of MN than non-smokers. A pooled re-analysis of 24 databases from the HUMN international collaborative project has been performed with the aim of understanding the impact of smoking habits on MN frequency. The complete database included 5710 subjects, with 3501 non-smokers, 1409 current smokers, and 800 former smokers, among subjects in occupational and environmental surveys. The overall result of the re-analysis confirmed the small decrease of MN frequencies in current smokers (frequency ratio (FR) = 0.97, 95% confidence interval (CI) = 0.93-1.01) and in former smokers (FR = 0.96, 95% CI = 0.91-1.01), when compared to non-smokers. MN frequency was not influenced by the number of cigarettes smoked per day among subjects occupationally exposed to genotoxic agents, whereas a typical U-shaped curve is observed for non-exposed smokers, showing a significant increase of MN frequency in individuals smoking 30 cigarettes or more per day (FR = 1.59, 95% CI = 1.35-1.88). This analysis confirmed that smokers do not experience an overall increase in MN frequency, although when the interaction with occupational exposure is taken into account, heavy smokers were the only group showing a significant increase in genotoxic damage as measured by the micronucleus assay in lymphocytes. From these results some general recommendations for the design of biomonitoring studies involving smokers can be formulated. Quantitative data about smoking habit should always be collected because, in the absence of such data, the simple comparison of smokers versus non-smokers could be misleading. The sub-group of heavy smokers (> or =30 cigarettes per day) should be specifically evaluated whenever it is large enough to satisfy statistical requirements. The presence of an interaction between smoking habit and occupational exposure to genotoxic agents should be always tested.

In vivo photochemical micronucleus induction due to certain quinolone antimicrobial agents in the skin of hairless mice

The skin micronucleus test combined with irradiation due to a sunlight simulator having a spectrum almost identical to solar irradiation was used as a novel in vivo testing method for detecting or comparing the photochemical chromosome damage of quinolone antibacterial agents (quinolones). Eight-week-old male SKH1 hairless mice were orally administered once lomefloxacin (LFLX), a strong in vitro photochemical clastogen, at 25 or 50 mg/kg, followed by light irradiation at 7.9–9.4 J/cm 2 of ultraviolet A (UVA). Animals were killed on Days 2, 3, 4, 5 or 8 (the dosing day was designated as Day 1), and the incidence of micronucleus in the epidermis was determined. As results, LFLX at either dose caused significant increases in the micronucleus frequency, which peaked on Day 4. These changes tended to return to the control level on Day 8. Then, the micronucleus induction potential of the quinolone derivatives levofloxacin (LVFX) and clinafloxacin (CLFX) at 10, 20 or 40 mg/kg was assessed on Day 4 under the same experimental conditions as for LFLX. Although LVFX was negative even at 40 mg/kg, CFLX dose-dependently induced significant increases in micronucleus frequency at all doses. The correlation of magnitude among the three quinolones in the skin micronucleus test with light irradiation was similar to that in our previous in vitro photochemical clastogenicity study. No significant increase in micronucleus frequency was observed in any of three quinolones employed without light irradiation. In conclusion, the experimental method presented here would be a useful tool for detecting in vivo photochemical chromosome damage and for research on photochemical carcinogenesis of chemicals.

Induction of micronuclei by alkaloids extracted from Senecio brasiliensis and stored for 23 years

In the present study, we report the results of an investigation on pyrrolizidine alkaloids extracted from Senecio brasiliensis (Sprengel) Less., which were stored for more than 23 years under variable conditions of temperature and humidity and exposed to light. Both the crude alkaloid (integerrimine+retrorsine+impurities) and pure integerrimine conserved the ability to induce acute toxicity in mice, leading to the death of the animals in less than 24 h. The alkaloids also conserved the potential to induce significant increases in micronucleus frequencies in polychromatic erythrocytes of mouse bone marrow compared to the negative control. The administration of alkaloids to lymphocyte cultures blocked with cytochalasin-B showed no significant increase in micronucleus frequency in binucleated cells, probably due to the lack of a metabolic activation mechanism. However, an antimitotic effect was observed.

Cytotoxic and genotoxic effects of fumonisin B1 on rabbit kidney RK13 cell line.

Fumonisins, mycotoxins produced by certain strains of Fusarium moniliforme, could induce various diseases in animals and are suspected human carcinogens. Fumonisin B1 (FB1), the most commonly found fumonisin, has been characterised as a tumour initiator and a tumour promoter, a mitogen and an anti-proliferative agent. In this study we examined the cytotoxicity and genotoxicity of FB1 in rabbit kidney RK13 cells. To evaluate the effects of FB1 on survival of this cell line we analysed cell viability, membrane integrity, DNA fragmentation and overall morphology of the cells. The genotoxic potential of FB1 was estimated by monitoring the ability of this mycotoxin to induce micronuclei in RK13 cells. Exposure to FB1 caused a significant increase in micronucleus frequency in a concentration- and in a time-dependent manner. Nanomolar concentrations of FB1 decreased cell viability after 24 h and even more so after 48 h of exposure. The morphological changes observed suggested that an increased number of RK13 cells were dying by the process of apoptosis. However, FB1 also induced impairments of cell and mitochondrial membrane integrity, as assessed by lactate dehydrogenase and glutamate dehydrogenase leakage. These results could imply that nanomolar concentrations of FB1 induced apoptosis, which subsequently may proceed to secondary necrosis. In summary, our observations suggest that FB1 is genotoxic and cytotoxic to RK13 cells.

P53 codon 72 polymorphism in coronary artery disease: no evidence for association with increased risk or micronucleus frequency.

A common polymorphism at codon 72 (Arg72Pro) of the p53 gene, a gene which codes for a tumor-suppressor protein with both antiproliferative and pro-apoptotic actions, has recently been reported to be a risk factor for coronary luminal narrowing after angioplasty. However, the association of the polymorphism with coronary artery disease (CAD) risk has not been studied. We evaluated the distribution of the Arg72Pro genotype in 250 patients, 180 with angiographically documented CAD and 70 with normal coronary angiography, by using polymerase chain reaction amplification of patient DNA followed by restriction enzyme digestion. We also examined the association between the Arg72Pro genotype and chromosome damage in 82 male patients (60 CAD and 22 no-CAD) by the micronucleus (MN) test in human lymphocytes, a sensitive assay for chromosome breakage and aneuploidy. The frequencies of Pro/Pro, Pro/Arg, and Arg/Arg genotypes in CAD patients were not significantly different from those who were CAD-free (chi(2) = 0.20, P = 0.90) and not significantly associated with the extent and severity of CAD. A significant increase in MN frequency was observed in relation to smoking status (8.4 +/- 0.6, 11.9 +/- 1 and 12.0 +/- 1.6, for non smokers, ex-smokers and smokers, respectively; P = 0.02). Moreover, diabetic patients showed higher levels of MN than normal patients (13.5 +/- 1.4 vs. 9.6 +/- 0.5, P = 0.0025). Also, MN frequency was significantly higher in CAD patients than in no-CAD patients (11.2 +/- 0.7 vs. 8.0 +/- 0.9, P = 0.02) and increased with the number of affected vessels (9.3 +/- 0.1, 12.2 +/- 1.5 and 12.5 +/- 1.3 for one-, two-, and three-vessel disease, respectively; P = 0.02). However, there were no associations between MN frequency and the Arg72Pro polymorphism. Although there appears to be an association between CAD and MN frequency, our results indicate that the Arg72Pro polymorphism does not have a significant impact on CAD or MN frequencies.

Effects of heat shock protein 70 (Hsp70) on arsenite-induced genotoxicity.

Arsenic, a human carcinogen, is genotoxic, although its mechanism(s) of action for tumorigenesis is not well understood. Among the toxicity-related properties of this chemical are its clastogenic and aneugenic activities, as well as its capacity for inducing stress-response in the form of elevated heat shock protein (HSP) expression. In the present study, we evaluated the effects of Hsp70 expression on arsenite (As)-induced structural and numerical chromosome anomalies in human cells. Human MCF-7 Tet-off cells stably transfected with a pTRE/Hsp70-1 transgene construct were used to regulate Hsp70 levels prior to in vitro As exposures. Separate cultures of relatively high vs. low Hsp70-expressing cells were established. A cytokinesis block micronucleus assay with kinetochore immunostaining was used to detect micronuclei (MN) derived from chromosome breakage (K-MN) or loss (K+MN). These studies demonstrated significant increases in micronucleus frequencies in response to As following either a long exposure (5 or 10 microM for 46 hr), or short exposure (10 or 40 microM for 8 hr) protocol. Overall, the long protocol was more efficient in producing K+MN and cells with multiple MN. Overexpressing Hsp70 resulted in significant reductions in the percent of cells positive for MN for both the long and short As exposure protocols. Both K+ and K- types of As-induced MN were lower in cells with elevated Hsp70 as compared to cells without overexpression of Hsp70. We conclude that the dose and duration of As exposure influence the type as well as amount of chromosomal alteration produced and that inducible Hsp70 protects against both the clastogenic and aneugenic effects of this chemical.

COMBINED EXPOSURE TO EXTREMELY LOW FREQUENCY (ELF) MAGNETIC FIELDS AND CHEMICAL MUTAGENS: LACK OF GENOTOXIC EFFECTS IN HUMAN LYMPHOCYTES

The aim of this study was to investigate the induction of genotoxic and cytotoxic effects in cultured human lymphocytes from healthy donors following combined exposure to a 50 Hz sinusoidal magnetic field (1 mT field intensity) and a chemical mutagen: mitomycin-C or hydroquinone. The induced effects were evaluated by applying the cytokinesis-block technique, because it offers the advantage of providing simultaneously information on both cell cycle progression (cytokinesis block proliferation index) and chromosomal damage (micronucleus frequency). The results obtained indicate that magnetic field exposure does not affect spontaneous micronucleus frequency, although a slight increase in cell proliferation was observed p = 0.031). Both mutagens induce a statistically significant increase in micronucleus frequency p = 0.0 and p = 0.016 for mitomycin-C and hydroquinone, respectively). Moreover, the combined exposure to both magnetic field and chemicals does not increase the damage due to the chemical treatment...

Evaluation of occupational genotoxic risk in a Brazilian hospital

Many therapeutic, diagnostic and prophylactic procedures used in hospitals are of potential genetic risk. An evaluation was made of genotoxic occupational risk in 42 workers from the Hospital de Clínicas de Porto Alegre, RS, Brazil, who had been occupationally exposed to lead (solder), ethylene oxide (sterilization area), antineoplastic drugs (nurses and pharmacists) or ionizing radiation. They were compared with 42 unexposed individuals. There was an increase in the frequency of binucleated cytochalasin-blocked lymphocytes with micronuclei, though it was not significant (P = 0.058). The groups exposed to antineoplastic drugs and radiation had a significant increase in micronuclei frequency (P = 0.038 and P = 0.022, respectively). The high frequencies of dicentric bridges suggest the action of clastogenics in these two groups. These results suggest that the safety procedures adopted were very important to protect workers from exposure to mutagenic agents and should be improved in the radiological and chemotherapeutical areas.

Combined effects of tirapazamine and mild hyperthermia on anti-angiogenic agent (TNP-470) treated tumors—reference to the effect on intratumor quiescent cells

Purpose: To evaluate the efficacy of the use of tirapazamine (TPZ), especially combined with mild hyperthermia (40°C, 60 min), in the treatment of solid tumors following an anti-angiogenic treatment with TNP-470. In addition, we assessed the effect of TPZ and/or mild hyperthermia (MHT) combined with conventional radiotherapy or chemotherapy on TNP-470 treated tumors. Materials and Methods: C3H/He mice bearing SCC VII tumors subcutaneously received TNP-470 at two doses of 100 mg/kg after tumor cell inoculation. At the same time, the tumor-bearing mice received 5-bromo-2′-deoxyuridine (BrdU) continuously for 5 days via implanted mini-osmotic pumps to label all proliferating (P) cells. The mice then received TPZ administration combined with or without MHT, γ-ray irradiation combined with or without TPZ and/or MHT, or cisplatin injection with or without TPZ and/or MHT. Another group of mice received a series of test doses of γ-rays while alive or after being killed to obtain hypoxic fractions (HFs) in the tumors at various time points after the above-mentioned cytotoxic treatment point. After each treatment, the tumors were excised, minced, and trypsinized. The tumor cell suspensions thus obtained were incubated with cytochalasin-B (a cytokinesis blocker), and the micronucleus (MN) frequency in cells without BrdU labeling (or quiescent [Q] cells) was determined using immunofluorescence staining for BrdU. The MN frequency in the total (P + Q) tumor cells was determined from the tumors that were not pretreated with BrdU. For the measurement of the HFs, the MN frequency of BrdU-unlabeled cells was then used to calculate the surviving fraction of the unlabeled cells from the regression line for the relationship between the MN frequency and the surviving fraction of total tumor cells. Results: TPZ administration combined with TNP-470 treatment and MHT increased the MN frequency more markedly than treatment with TPZ alone, and this tendency was more remarkable in Q cells than total cells. In both total and Q cells, combined treatment with TPZ and MHT produced significant increases in MN frequencies whether γ-rays were delivered to TNP-470 treated tumors or cisplatin was injected into the TNP-470 administered mice. Although not significantly, the HFs of total and Q cell populations within solid tumors increased after TNP-470 treatment. Conclusion: Combined treatment with TPZ and MHT, whether other cytotoxic treatments such as γ-ray irradiation or chemotherapy using cisplatin were combined or not, was useful for sensitizing tumor cells in vivo including Q cells even after TNP-470 treatment.

Induction of micronuclei formation in preimplantation mouse embryos after maternal treatment with 2-bromopropane

We examined effects of 2-bromopropane (2-BP), a chlorofluorocarbon replacement, on mouse embryonic mutagenicity. 2-BP was administered to pregnant mice intraperitoneally (i.p.) (300, 600, 900, and 1800 mg/kg) during the early preimplantation period. On day 3 of gestation, micronuclei (MN) frequency and embryo cell number were determined. 2-BP induced a dose-related significant increase in MN frequency and a treatment-related decrease in embryo cell number. Furthermore, the cell numbers were significantly smaller in the MN-positive embryos by two-way ANOVA taking it into account an interactive effect between 2-BP dose and the presence or absence of MN. A simultaneous decrease in cell number and increase in MN frequency may reflect an embryonic developmental disadvantage resulting from maternal treatment with 2-BP. Further study is needed to establish how 2-BP contributes to postimplantation embryonic development.

Induction of micronuclei by CsCl in vivo and in vitro

In the present study, we report the results of an investigation of the potential of nonradioactive CsCl for the induction of micronuclei in polychromatic erythrocytes of mouse bone marrow and in human lymphocytes cultured and blocked with cytochalasin-B. No significant increase in micronucleus frequency was observed in the polychromatic erythrocytes of mice which received 500 mg/kg of CsCl. In vitro experiments with human lymphocytes cultured in medium containing 250 and 500 μg/ml CsCl also showed no increase in micronucleus frequency compared to untreated controls. These same experiments, however, demonstrated a reduction in mitotic activity with increasing CsCl concentration in the culture medium. This report is the first to describe studies on the possible induction of micronuclei in vitro and in vivo by nonradioactive CsCl.

Increased genomic damage in lymphocytes of patients before and after long-term maintenance hemodialysis therapy.

This study investigates spontaneous genomic damage in peripheral lymphocytes of 19 patients with severe end-stage renal disease not enrolled onto a maintenance hemodialysis (MHD) program (creatinine level, 5.4 to 10.5 mg/dL) and 16 long-term MHD patients (111 to 282 months on MHD) and the possible association of genomic damage with the degree of renal insufficiency and duration of MHD. Genomic damage was assessed by evaluating the numbers of micronuclei (MN), which are cytoplasmic DNA-containing structures. The average number of MN in the control group of 23 healthy subjects was 15.3 +/- 4.7 MN/1,000 binucleate (BN) cells. The MN frequencies in the long-term MHD group were significantly greater (44.3 +/- 13.7 MN/1,000 BN) than the control frequencies. A significant increase in MN frequencies (28.2 +/- 9.4 MN/1,000 BN) was also seen in patients with advanced renal failure. The major findings of the study, high MN frequencies in long-term hemodialysis and advanced chronic renal failure patients, may result from decreased DNA repair previously shown and may contribute to the increased cancer incidence in these patients.

Absence of synergistic effects on micronucleus formation after exposure to electromagnetic fields and asbestos fibers in vitro

Exposure of human amniotic fluid (AFC) cells to horizontally applied magnetic fields (hMF) of 50 Hz and 1 mT generated in a Helmholtz-coil system leads to a significant increase in micronucleus frequency (MN), without affecting cell proliferation. To investigate whether hMF-exposure has an additive or synergistic effect on the genotoxic capacity of asbestos fibers, MN induction was investigated in hMF pre-exposed cells, treated before or after with asbestos (1 μg/cm 2). Neither synergistic nor additive effects on MN induction were observed. The results indicate, that under our experimental conditions, exposure to hMF and treatment with asbestos fibers possess genotoxic capability, but no interactive effects, in AFC cells.

Flow cytometric measurement of micronuclei induced in a permanent fish cell line as a possible screening test for the genotoxicity of industrial waste waters.

An in vitro micronucleus assay using the permanent fish cell line RTG-2 (rainbow trout gonads) was developed to test industrial waste waters for their genotoxic potential. Comparison of flow cytometric measurement and microscopic scoring of micronucleus frequency with the reference chemicals 1,4-butane sultone (0.2-1 mM), ethylmethane sulphonate (2-10 mM), potassium dichromate (20-100 microM) and benzo[a]pyrene (5-25 microM) showed similar dose-effect relationships. Thirty-eight industrial waste waters from 11 different branches of industry obtained from the Bavarian state office for water research were tested using the flow cytometric method (18 from metal processing, 10 from combined waste water, two from synthetic fibre production, one sample each from settlement wastes, non-iron metal manufacturing, leather production, sulphuric acid production, ore processing, graphite film production, cellulose production and flue gas washing). Fourteen of them showed a significant increase in micronucleus frequency.

Monitoring of mutagens in urban air samples

This research was designed to examine the presence of mutagenic/carcinogenic compounds in urban airborne particulates sampled with the inhalable PM-10 high volume sampler in two different streets of Brescia, a heavily industrialized town in northern Italy, using the Tradescantia/micronucleus test and a bacterial mutagenicity test (Kado test, a more sensitive version of the Ames test). In addition, the Tradescantia/micronucleus test was used for in situ monitoring of gaseous pollutants in other urban areas of Brescia and in two car tunnels, one with heavy car traffic in Perugia, a town in central Italy, and one in Brescia with moderate traffic. The Tradescantia-micronucleus test carried out on extracts of airborne particulates gave positive results only for the sample collected in the traffic-congested street where also higher bacterial mutagenicity was found. The in situ monitoring of the urban areas with the Tradescantia/micronucleus test always gave negative results. Monitoring carried out in the two car tunnels showed a significant increase in micronuclei frequency only in flowers exposed in the smaller and more polluted tunnel.

Assessment of the utility of the micronucleus test for petroleum-derived materials

The micronucleus test is a commonly used in vivo assay for chromosomal damage and is an integral part of many mutagenicity testing strategies. The present report describes an assessment of the micronucleus test for the detection of mutagenic potential of petroleum-derived materials. To this end, studies were conducted with catalytically cracked clarified oil (CCCO). This material contains high levels of polycyclic aromatic constituents (PAC) and is a very potent inducer of mouse skin tumors. CCCO is also active in the Salmonella assay and other in vitro tests. As CCCO is the most potent of the various petroleum-derived materials in other assays, it was assumed to be the most easily detectable in the micronucleus test. CCCO was tested in standard mouse micronucleus tests utilizing oral and intraperitoneal injection for test material administration. All of these studies were negative, although DMBA, tested at roughly equivalent levels based on potency in the Salmonella assay, produced statistically significant increases in micronucleus frequency. In a second series of studies, aromatic fractions of CCCO were prepared and tested at up to acutely toxic levels. Results of these studies were also negative. Finally, another petroleum-derived material which is carcinogenic and contained PAC was tested in the micronucleus assay. It also produced negative results. Thus, it was concluded that petroleum-derived materials do not produce clastogenic effects in vivo in the mouse micronucleus test, despite the fact that some pure polycyclic aromatic hydrocarbons are quite active in this assay.

Micronucleus formation in human amnion cells after exposure to 50 Hz MF applied horizontally and vertically

Micronucleus (MN) induction as a genotoxic effect of extremely-low-frequency electromagnetic fields (ELF-EMF, 50 Hz, 1 mT) was studied in human amniotic fluid cells (AFC) after continuous exposure to magnetic fields (MF), oriented horizontally and vertically with respect to the surface of the culture medium, at different time points. To compare the effectiveness of different exposure systems, a Helmholtz-coil system and a so-called Merritt-coil system was used. A statistically significant increase in MN frequency could be detected in exposed cells compared to controls after 72 h continuous exposure to MF applied vertically in the Merritt-coil system, while no effect was found after exposure in the Helmholtz-coil system. Furthermore, a significant increase in MN induction occurred after 24, 48 and 72 h exposure to MF applied horizontally in the Helmholtz-coil system in comparison to controls, whereas horizontally MF generated in the Merritt-coil system induced no genotoxic effects. To exclude suppression of indirect EMF-induced DNA-lesions, we studied MN formation in the presence of N-Acetyl- p-aminophenol (APAP, Paracetamol®), which is an inhibitor of DNA-repair mechanisms. We found a dose-dependent increase of MN formation in APAP-treated AFC cells, but no significant further increase in MN frequency after additional MF exposure. Therefore we conclude, that EMF-induced MN formation is not caused by directly or indirectly induced clastogenic mechanisms. The obtained results show that the orientation of MF with respect to the cell culture dish and the physical condition of the exposure system is of major importance for the induction of micronuclei in certain cell types. Therefore, the reason for inconsistent results published in the literature may be caused by the variability of exposure systems, the exposure conditions and the cell types used.

Influence of vindesine exposure on the micronucleus formation and cell survival in V79 cells

Effect of different concentrations (0, 1, 5, 10, 20 and 50 nM) of vindesine sulphate was studied on clonogenicity and micronucleus (MN) formation in V79 (Chinese hamster lung fibroblasts) cells. Exposure of V79 cells to vindesine for 6 h resulted in a concentration dependent decline in cell survival. The frequency of micronuclei (MN) increased in a concentration dependent manner at 16, 22 and 28 h post-exposure. The frequency of MN increased significantly after 5 to 50 nM drug exposure at 16 and 22 h post-treatment, while increasing post-exposure time to 28 h resulted in a significant increase in MN frequency at all exposure doses of vindesine. The statistical evaluation of concurrent concentrations at various time periods showed a non-significant difference in MN frequency among various post-exposure time periods, except 16 h and 28 h for 50 nM, where a significant decline in the MN frequency was observed at 28 h compared to 16 h post-exposure. The cell proliferation indices showed a concentration dependent decline in the frequency of binucleate cells and this decline was linear quadratic. The increasing drug concentration resulted in a concentration dependent decline in cell survival. While the frequency of micronuclei increased the cell survival decreased and the relationship between cell survival and micronucleus induction was linear quadratic.

Effects of 50 Hz EMF exposure on micronucleus formation and apoptosis in transformed and nontransformed human cell lines

Effects of applying extremely low-frequency electromagnetic fields (ELF-EMF) for different durations (24, 48, and 72 h) and different field intensities (0.1–1.0 mT) on micronucleus (MN) formation and induction of apoptosis were examined in a human squamous cell carcinoma cell line (SCL II) and in a human amniotic fluid cell line (AFC). A statistically significant increase of MN frequency and of induction of apoptosis in SCL II cells after 48-h and 72-h continuous exposure to 50 Hz magnetic field (MF) (0.8 and 1.0 mT) was found. However, exposure of AFC cells to EMF of different intensities and for different exposure times showed no statistically significant differences when compared with controls. These results demonstrate that different human cell types respond differently to EMF. Dose-dependent induction of apoptosis and genotoxic effects, resulting in increased micronucleus formation, could be demonstrated in the transformed cell line, whereas the nontransformed cell line did not show statistically significant effects. These findings suggest that EMF could be a promotor but not an initiator of carcinogenic effects. Bioelectromagnetics 19:85–91, 1998. © 1998 Wiley-Liss, Inc.

Studies on the genotoxicity of molybdenum salts in human cells in vitro and in mice in vivo.

Molybdenum is an essential element in plants and animals as a cofactor for enzymes. Molybdenum trioxide is used in metallurgical processes, in cosmetics as a pigment, and in contact lens solution, yet limited information is available on molybdenum genotoxicity. In the present study the micronucleus (MN) assay in human lymphocytes and mouse bone marrow and the dominant lethal assay in mice were used to assess the genotoxic effects of molybdenum salts in vitro and in vivo. Two salts of molybdenum were tested in whole blood cultures. Ammonium molybdate was more potent than sodium molybdate in causing a dose-dependent decrease in viability and replicative index and an increase in MN formation in binucleated lymphocytes (P < 0.001). A dose-response in both kinetochore-positive MN (caused by chromosome lagging) and kinetochore-negative MN (associated with chromosome breakage) was observed. Based on the results of a toxicity study of sodium molybdate, two doses, 200 and 400 mg/kg, were assessed in the bone marrow MN assay in mice (two i.p. injections 24 and 48 hr prior to euthanasia). A modest but statistically significant increase in MN frequency in polychromatic erythrocytes was observed (P < 0.05). The same treatment protocol was used to analyze dominant lethality. A dose-dependent increase in postimplantation loss represented mostly by early resorptions was observed the first week after treatment (P = 0.003). These preliminary data suggest that sodium molybdate induces dominant lethality at the postmeiotic stage of spermatogenesis. Overall, molybdenum salts produced moderately positive results both in vitro in human cells and in vivo in mice.