Log Dose-response Relationship Research Articles

Approaches to Gradual Dose Reduction of Chronic Off-Label Antipsychotics Used for Behavioral and Psychological Symptoms of Dementia.

Little is known about how to best taper antipsychotics used in patients with dementia. To address this gap, we reviewed published antipsychotic discontinuation trials to summarize what is known about tapering strategies for antipsychotics used with older adults with dementia. We further developed pharmacokinetic-based gradual dose reduction (GDR) protocols based on antipsychotic half-lives. MEDLINE, EMBASE, and International Pharmaceutical Abstracts were searched up to October 2014 to identify intervention studies reporting the behavioral and psychological symptoms of dementia outcomes resulting from discontinued off-label use of antipsychotics in nursing facility populations. Recently published pharmacokinetic reviews and standard pharmacology texts were used to determine antipsychotic drug half-lives for the pharmacokinetic-based GDR protocols. For the review, studies with an intervention resulting in antipsychotic medication discontinuation or tapering were eligible, including randomized controlled trials and pre- and post-intervention studies. When available, we extracted the protocols used for antipsychotic GDR from each study included in the review. We found that clinical trials used different approaches to antipsychotic discontinuation, including abrupt discontinuation, slow tapers (more than two weeks), and mixed strategies based on drug dosage. None of the published trials described an approach based on pharmacokinetic principles. We developed a two-stage GDR protocol for tapering antipsychotic medications based on the log dose-response relationship; each stage was designed to result in a 50% dose reduction prior to discontinuation. This pharmacologically based strategy for patients chronically prescribed antipsychotics resulted in recommendations for slow tapers. Our theoretically derived GDR recommendations suggest a different approach than previously published in clinical trials. Further study is needed to evaluate the effect of this approach on patients.

Are Homeopathic Remedies Effective?

Are Homeopathic Remedies Effective?

Use of solid phase microextraction to estimate toxicity: Relating fiber concentrations to toxicity—part I

Use of solid-phase microextraction (SPME) fibers as a dose metric for toxicity testing was evaluated for hydrophobic pesticides to the midge Chironomus dilutus and the amphipod Hyalella azteca. Test compounds included p,p'-dichlorodiphenyltrichloroethane (p,p'-DDT), p,p'-dichlorodiphenyldichloroethane (p,p'-DDD), p,p'-dichlorodiphenyldichloroethylene (p,p'-DDE), permethrin, bifenthrin, tefluthrin, and chlorpyrifos. Acute water toxicity tests were determined for 4- and 10-d exposures in both species. Median lethal and sublethal concentrations were expressed both on a water concentration (LC50 and EC50) and on an equilibrium SPME fiber concentration (LC50(fiber) and EC50(fiber)) basis. A significant log dose-response relationship was found between log fiber concentration and organism mortality. It has been shown in the literature that equilibrium SPME fiber concentrations reflect the bioavailable concentrations of hydrophobic contaminants, so these fiber concentrations should be a useful metric for assessing toxic effects from the bioavailable contaminant providing a framework to expand the use of SPME fibers beyond estimation of bioaccumulation.

Plasma concentrations of clobazam and its N-desmethyl metabolite; protection against pentetrazol-induced convulsions in mice.

The anticonvulsant effects of acute administration of clobazam and its principal metabolite N-desmethylclobazam were studied in mice. Pentetrazol, given by slow intravenous infusion 1 or 2 h after the anticonvulsant dose, was used as the convulsant stimulus. Log dose response relationships for both clobazam and N-desmethylclobazam appeared linear, but there was no correlation between plasma concentrations of clobazam and protection. However, correlation between plasma concentrations of N-desmethylclobazam and protection was significant in both cases.

Correlation of mechanistic data and histopathology in the evaluation of selected toxic endpoints of the endocrine system

The objective of this review is to correlate endocrinologic data from mechanistic studies with quantitative histopathology in selected examples of toxic endpoints of the endocrine system in laboratory animals. Mechanistic data can aid in the interpretation of animal toxicology findings and help clarify their significance in risk assessment. Endocrine organs of rodents frequently undergo proliferative changes with advancing age and following chronic exposure to large doses of xenobiotic chemicals, and the sensitivity of rodent endocrine tissues appears to be increasing. Many xenobiotic chemicals in large doses disrupt thyroid function in rodents either by a direct effect on the thyroid influencing synthesis of thyroid hormones or by adversely influencing their peripheral metabolism. A number of chemicals disrupt thyroid function by inhibiting the important enzyme, thyroperoxidase (TPO). A contemporary example of a chemical acting as TPO-inhibitor is sulfamethazine. In short-term mechanistic studies in rats there was a log–dose response relationship in circulating levels of thyroid and pituitary hormones plus a similar non-linear dose-response in morphologic changes in thyroid follicular cells. Endocrinologic data from mechanistic studies and histopathologic/ultrastructural findings will also be presented for the effects of the food color, FDC Red No. 3 (Erythrosine), on the thyroid gland in rats and parathyroid hormone-related protein (a major causative factor in cancer-associated hypercalcemia) on parathyroid chief cells in mice.

In vitro bioassay for human erythropoietin based on proliferative stimulation of an erythroid cell line and analysis of carbohydrate-dependent microheterogeneity

The human erythroleukemia cell line TF-1 was employed for the determination of proliferative stimulation induced by recombinant human erythropoietin (rhEpo). Potencies of various intact and sugar-trimmed rhEpo preparations were estimated using the International Standard for Human r-DNA-derived Epo (87/684) as a reference for activity. The cellular response was measured in a multi-channel photometer using a colorimetric microassay, based on the metabolism of the tetrazolium dye 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide to formazan, by viable cells. The linear part of the log dose-response relationship encompassed 2.5–90 pM and activity of rhEpo preparations was measured at doses between 3 and 60 pM. The assay was designed as a parallel line test, using three or four concentrations for potency determinations, which fulfills pharmacopoeial requirements for assay validity. Inter-assay relative standard deviation varied between 4.1% and 12.6% and most assays revealed potencies with limits of error within 87–113%. In order to acquire an additional means for an efficient probing of physiologically relevant features of rhEpo, a luminiscence-dependent Western detection system, based on a combined isoelectric focusing/sodium dodecyl sulphate-polyacrylamide gel electrophoresis separation, was established. As opposed to conventional electrophoresis the two dimensional approach enabled the disclosure of minor truncations in the rhEpo-attached glycan moieties using picomolar quantities of the hormone. Moreover, the separated isoforms of rhEpo were quantified by computer-assisted densitometry and compared with the 87/684 standard. Accordingly, results obtained by the cellular response were balanced against the general pattern observed and the relative amounts of separated rhEpo isomers as determined by the quantitative Western analysis. The method described should be suitable for potency assessments of pharmaceutical formulations of rhEpo.

Stria terminalis conveys a facilitatory estrogen effect on female rat lordosis reflex

The effects of bilateral cut of the stria terminalis (ST) on the dosage of estradiol benzoate (EB) needed to induce the lordosis reflex in the ovariectomized female rat were examined. Seven different doses ranging 0.5–20 μg were given s.c. in oil, daily for 2 days in a counterbalanced order, at 3-week intervals. The log dose-response relationship was linear in both ST-cut and sham-operated control rats. A significantly larger dose of 9.3 ± 1.4 (mean ± SEM) μg/day of EB was needed in the ST-cut rats ( n = 8) to obtain a 50% lordosis quotient than in the controls ( n = 7), in which a 2.0 ± 0.3 μg/day dose had a similar effect. The slope of the dose-effect curve was significantly steeper in the ST-cut rats than in the controls. The ST mediates facilitatory effects of estrogen on the lordosis reflex that originate presumably in the amygdala.

Bioassay from two parabolas

The paper deals with potency ratio estimation of parallel curve analytic dilution assays in case log dose-response relationship could be reasonably described by a parabola. It comprises: (1) testing the adequacy and validity of the quadratic model by the analysis of variance; and (2) estimation of the relative potency of an unknown in relation to a standard preparation, its standard deviation and fiducial limits for its true value. The method is applicable whenever successive doses of an unknown are a constant multiple of a standard in randomized blocks or completely randomized design. The method can be generalized to polynomial models of higher order.

Intravitreal Aminoglycoside Toxicity Revisited-Reply

In Reply. —We appreciate the interest and concern of Dr Pulido et al regarding the determination of aminoglycoside toxicity following intravitreal administration. Basic toxicity studies are performed for safety assessment so we can predict the likely adverse effects of a drug in humans. Two objectives of toxicity studies include defining a nontoxic (no observable effect) dose level and a toxic dose level. The characterization of the dose-response relationship for toxic effects observed at doses above the no-effect level is vital. Two approaches can be undertaken to achieve the objectives: incremental increases in the drug-dose level, and a log-dose interval, as suggested by Dr Pulido et al. An experimental data set with a dose-response relationship over several log units of drug dose may allow reliable interpretation of the results and perhaps extrapolation of the results to humans, as stated by Dr Pulido et al. The log doseresponse relationship is critically important

Adrenergic receptors and the regulation of vascular resistance in bullfrog tadpoles (Rana catesbeiana).

Vascular adrenergic sensitivity to exogenous catecholamines was examined in tadpoles of the American bullfrog (Rana catesbeiana), ranging from stage III to XIV. Central arterial blood pressure was measured in decerebrate bullfrog tadpoles to determine a reasonable initial infusion pressure. Solutions of epinephrine and phenylephrine were infused into the vasculature of pithed tadpoles, and the resulting changes in vascular resistance (Rv) were used to construct log dose-response relationships. Epinephrine infusion produced a dose-dependent increase in Rv (EC50 = 5.3 x 10(-7) M), which could be reversed by sodium nitroprusside (a smooth muscle relaxant) and blocked by phenoxybenzamine (an alpha-adrenergic antagonist). Larval Rv also increased with infusion of the alpha-agonist phenylephrine (EC50 = 7.4 x 10(-8) M). Infusion of 10(-6) M isoproterenol (a beta-agonist) largely reversed the phenylephrine-induced increase in Rv. These results indicate that the capacity exists for both alpha-mediated vasoconstriction and beta-mediated vasodilation early in bullfrog ontogeny. Neither initial Rv nor the responses to infused epinephrine or phenylephrine were significantly correlated to development over the range of larval stages used in this study.

Standard and high dose chemotherapy for advanced soft tissue sarcomas.

While significant improvements have been made in the management of localized soft tissue sarcomas, enabling the realization of better functional results and a better overall outcome, progress in metastatic disease has been less than impressive. Adriamycin and DTIC based chemotherapy programs have resulted in response rates of upto 50%, with a small but finite cure fraction, especially in conjunction with surgical resection of residual abnormalities. The decade of the 80s experienced a considerable amount of enthusiasm in exploring the role of ifosfamide and mesna, and identified its definite usefulness as an effective salvage regimen with response rates of approximately 25%-30% in adriamycin failures. Studies evaluating the role of ifosfamide as up-front line agent in combination with adriamycin have met with increased toxicities without a significant additive therapeutic benefit, probably as a result of compromised dose intensity of each individual agent. The steep linear log-dose response relationship of alkylating agents and adriamycin constitutes the theoretical rationale for use of higher doses of chemotherapy, which have become feasible, with the advent of growth factors for bone marrow support. While this approach seems promising in its infancy, with improved complete and overall response, its ultimate effect on survival is anxiously awaited. The limited experience available in the literature with even more aggressive approaches like marrow ablative doses of chemotherapy followed by bone marrow transplantation, have been uniformly disappointing, with extremely short lived responses and extremely high morbidity and cost. As clinical research continues to improve our understanding and ability to implement the currently available therapeutic armamentarium, the search for newer and better drugs needs to continue.(ABSTRACT TRUNCATED AT 250 WORDS)

The Quantification of Steroidogenesis- Stimulating Activity in Testicular Interstitial Fluid by anin VitroBioassay Employing Adult Rat Leydig Cells*

An in vitro bioassay for steroidogenesis-stimulating activity (SSA) in charcoal-extracted rat testicular interstitial fluid (IF) was developed. The bioassay was based upon stimulation of testosterone production by Percoll gradient-purified adult rat Leydig cells during a 20 h incubation in the presence of a maximally stimulating dose of human CG (hCG). The hCG-stimulated conditions were employed to avoid assay interference by endogenous LH in the sample preparations. The standard preparation (a pool of IF from normal adult rats) stimulated testosterone production 3- to 4-fold above that obtained with a maximal dose of either hCG or LH alone, and a linear log dose-testosterone response was observed over the range from 150% to 300% of hCG-stimulated testosterone production. The bioassay had a mean index of precision (lambda) of 0.13 (n = 10 assays), a between assay variation of 10.7-11.7%, and a useful working range from 4.9-28 microliters IF including at least three serial half-dilutions. Parallelism with the IF standard was obtained with IF collected from aged (greater than 15 months old) rats, adult rats made bilaterally cryptorchid for either 4 weeks or 12 months, or injected 6 h previously with 100 IU hCG, and with ovine testicular lymph. Testicular SSA was not affected by coincubation with rat LH antiserum and was not attributable to prevention of oxygen-mediated enzyme damage during the incubation period. Although charcoal-extracted rat serum log dose-response relationships were nonparallel with the standard, serum displayed an apparent relative bioactivity of approximately 20% that of normal IF based on ED50 comparisons. The activity of testicular IF was not affected by coincubation with serum. Untreated and charcoal-extracted rat albumin, at an assay concentration equivalent to that present in normal rat serum or IF, caused only a minor stimulation of testosterone production. Charcoal-extracted bovine albumin, ovalbumin, epidermal growth factor, insulin-like growth factor-1, bovine 31 kDa inhibin, and transforming growth factor-beta were inactive. Activity was nondetectable in rat thoracic duct lymph or high speed supernatants of adult rat testicular extracts. Testicular SSA and serum from normal rats displayed slightly different time-courses of action, with SSA active during both acute (1.0 h) and longer term (2.0-20 h) incubations, while serum stimulated testosterone production only in the longer term incubations.(ABSTRACT TRUNCATED AT 400 WORDS)

Quantitative analysis of antiatherosclerotic effect of nifedipine in cholesterol-fed rabbits

Reports concerning the effect of slow calcium-channel blockers on experimental atherosclerosis are controversial. We examined the antiatherosclerotic effect of nifedipine (40 mg/day for 16 weeks) on aorta of rabbits on diets containing 0.3%, 0.5%, and 1.0% cholesterol. There were no significant differences in levels of serum lipids with or without nifedipine in the same cholesterol-fed rabbits. The results obtained show that nifedipine suppressed the extent of lipid deposition and surface involvement (S.I.) in aorta in 0.3% cholesterol-fed rabbits, whereas nifedipine only tended to suppress S.I. in 0.5% cholesterol-fed rabbits and had no effect in 1.0% cholesterol-fed rabbits. The log dose-response relationship of S.I. was obtained by plotting the concentration of cholesterol in the feed or the "integrated value" of the total serum cholesterol (TC), i.e., the cumulative sum of the serum TC values obtained at each week. The log dose-response curve was shifted in parallel with the right in nifedipine groups. The Lineweaver-Burk plot constructed from the dose-response curve had the same points crossing the ordinate with or without nifedipine. These results suggested that nifedipine suppressed S.I. in a competitive manner with cholesterol on the specific binding site of lipid deposition. Electron-microscopic findings also demonstrated that fat droplets in smooth muscle cells, extracellular matrix containing collagen, and elastic fibers decreased in nifedipine-treated rabbits.

Biphasic response for hepatic microsomal enzyme induction by 2,3,7,8-tetrachlorodibenzo- p-dioxin in C57BL/6J and DBA/2J mice

The induction of the murine hepatic microsomal cytochrome P-450 monooxygenase system by 2,3,7,8-tetrachlorodibenzo- p-dioxin (TCDD) was studied over a wide range of doses, including those associated with acute toxicity. Studies were conducted in two inbred strains of mice which vary at the Ah receptor and at a number of other genetic loci. C57BL/6J mice possess at high-affinity Ah receptor and are responsive to enzyme inductive effects of TCDD, whereas DBA/2J mice do not possess a high-affinity receptor and are less responsive to TCDD. In a dose-response study, 7-ethoxyresorufin O-deethylase (EROD) activity appeared to be maximally induced in C57BL/6J and DBA/2J mice at 7 days following exposure to 3 and 30 μg of TCDD/kg respectively. Very similar results were reported previously for the induction of aryl hydrocarbon hydroxylase activity in these strains of mice. However, at higher doses of TCDD (at least 45 μg/kg for C57BL/6J and 300 μg/kg for DBA/2J), EROD activity was further increased (2-fold) from the apparent maximal (plateau) level, resulting in an unusual biphasic log dose-response relationship. EROD activity remained at these elevated rates in both strains for doses approaching and exceeding the respective ld 50 values for each strain. To further characterize this biphasic induction phenomenon, cytochrome P-450 content, benzo[ a]pyrene metabolism, and EROD and NADPH-cytochrome P-450 reductase activities were measured 1, 3 and 7 days after TCDD administration to C57BL/6J (3 and 150 μg/kg) and DBA/2J (30 and 600 μg/kg) mice. Maximal responses occurred in both strains at 3 days for all doses. In both strains, TCDD produced a dose-dependent increase in cytochrome P-450 content, EROD, and benzo[ a]pyrene metabolism. Furthermore, a 2-fold induction of reductase activity was observed in each strain following exposure to the respective high doses. Induction of cytochrome P 1-450 and P 3-450 was also measured by Western immunoblot, using antisera raised against the homologous rat isozymes. In both strains, TCDD produced a dose-related increase in two protein-staining bands recognized by anti-P-450 BNF-B (P 1-450) and anti-P-450 BNF/ISF-G (P 3-450) respectively. The extended induction of hepatic microsomal monooxygenase activities at the respective high doses of TCDD appears to be due, in part, to increases in NADPH-cytochrome P-450 reductase activity and cytochromes P 1-450 and P 3-450 content. Significant alterations in the expression of the cytochrome P-450 monooxygenase system following exposure to high doses of TCDD may be associated, in part, with the delayed acute toxicity reported at this level of exposure.

Differential development of acute tolerance to analgesia, respiratory depression, gastrointestinal transit and hormone release in a morphine infusion model

To determine whether the differences in development of acute tolerance to several morphine actions correlate with the mu receptor subtype mediating them, we have examined the appearance of acute tolerance to analgesia, respiratory depression, gastrointestinal transit, and hormone release in an intravenous morphine infusion model. Analgesia, a naloxonazine-sensitive mu1 action, peaked at 2 hr after initiation of the infusions. The log dose-response relationship of the infusion rate to peak tailflick latency was linear from 10 to 50 micrograms/kg/min. By 8 hr, the tailflick latencies declined nearly to baseline levels, implying the rapid development of tolerance. Tolerance to morphine-induced prolactin release, another mu1 action, also developed rapidly over 8 hr. In contrast two mu2 actions, respiratory depression measured with arterial blood gas, determinations and gastrointestinal transit, showed no significant tolerance over a similar 8 hr infusion. We also observed no tolerance to morphine-induced growth hormone release, a non-mu1 action, over the same period. Thus, these results demonstrate that mu1 actions develop tolerance in an infusion model far more rapidly than a number of naloxonazine-insensitive (non-mu1) ones and may help explain differences in the rate of tolerance development to morphine actions.

Beta-adrenoceptor responses to high doses of inhaled salbutamol in patients with bronchial asthma.

1. Fourteen asthmatics (mean +/- s.e. mean baseline FEV1 62 +/- 6% of predicted) were given cumulative doubling doses of salbutamol by metered-dose inhaler as follows: 100 micrograms, 200 micrograms, 500 micrograms, 1000 micrograms, 2000 micrograms, 4000 micrograms. 2. Airways, tremor, haemodynamic and cyclic AMP responses were measured at each dose increment (made every 20 min). 3. There was a linear log dose-response relationship for each airways parameter (FEV1, VC, sGaw, FEF 50%). The plateau in the dose-response curve was not reached within our dose range. These changes were also mirrored in cyclic AMP responses. 4. There was a wide range in maximum airways response expressed in terms of absolute increase over baseline (95% confidence intervals: delta FEV1 667-1483 ml; delta VC 689-1695 ml; delta sGaw 0.92-4.50 s-1 kPa-1; delta FEF 50% 0.94-2.15 l s-1). Patients with a lower baseline showed a greater response in terms of percent increase in FEV1 (r = -0.83, P less than 0.001). There was however, no correlation between baseline airway calibre and the dose required for maximum bronchodilatation. 5. There were objective increases (mean +/- s.e. mean) in both heart rate (maximum delta HR of 14 +/- 5 beats min-1 at 4000 micrograms) and tremor power (maximum delta Tr of 115 +/- 44% at 2000 micrograms). These were not dose limiting side-effects as subjective symptoms were infrequent at higher doses. 6. Higher than conventional doses of salbutamol given by metered-dose inhaler may produce a distinct improvement in airways response without significant side-effects.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparison of corticotropin-releasing factor and acetylcholine on catecholamine secretion in dogs.

To test whether corticotropin-releasing factor (CRF) is a secretagogue for adrenal secretion of catecholamines, a preparation was developed that permits measurement of adrenal venous output in response to in vivo arterial injection into the dog adrenal gland. Dogs were prepared with catheters in the lumboadrenal vein for monitoring adrenal blood flow and secretion rate of epinephrine and norepinephrine and in the lumboadrenal and inferior phrenic arteries for adrenal injection. Under pentobarbital anesthesia 48 h after surgery, dogs received a series of intra-adrenal injections that included acetylcholine (0.2-200 nmol), CRF (2.0-20.0 nmol), and diluent. There was a log dose-response relationship before epinephrine secretion and norepinephrine secretion to acetylcholine. Adrenal injection of CRF stimulated epinephrine and norepinephrine secretion at the highest dose tested (20 nmol). The response observed was equivalent to the response to 0.2 nmol acetylcholine. A similar dose of CRF given systemically produced hypotension without stimulating catecholamine responses. Adrenal catecholamine responses to acetylcholine were not augmented by addition of CRF. These findings show that arterial injection of CRF into the intact dog adrenal stimulates secretion of epinephrine and norepinephrine. However, the low potency of CRF relative to that of acetylcholine, the lack of a synergistic effect of CRF on catecholamine responses to acetylcholine, and the high dose of CRF required to achieve a response suggest that CRF does not function in the adrenal medulla as a physiologically important secretagogue for catecholamines.

FUROSEMIDE PHARMACODYNAMICS FOLLOWING ACUTE & CHRONIC ADMINISTRATION

Furosemide (F) pharmacodynamics in VLBW infants have not been described following initial and subsequent doses during chronic adminstration. F acts from the luminal side of the loop of Henle and there is a log dose-response relationship between diuretic responses (Na excretion rate, urine output) and F urinary excretion rate. Serial urine collections were performed on 8 VLBW infants begun on F for treatment of BPD. Urine volume and F and sodium concentrations were measured. A linear relationship between diuretic response and F excretion rate was noted in all patients. Natriuretic response (mEq/hr) to initial doses of F decreased significantly with increasing gestational age (p<.02). The F excretion rate required to produce a standard diuretic response was significantly greater (p=.05) during chronic administration than following the initial dose. No consistent pattern of maturation of diuretic response was noted during subsequent chronic administration. We conclude that apparent sensitivity to F decreases with increasing gestational age at the start of therapy and with subsequent chronic doses. These changes may reflect the ability of the renal tubule to compensate for diuretic induced sodium and water loss. Results of previous studies of furosemide pharmacodynamics in VLBW infants following single initial doses can not be extrapolated to chronic administration.

Prothoracicotropic hormone bioassay:Bombyxlarva assay

Summary An assay for the prothoracicotropic hormone (PTTH) has been established using in situ activation of the prothoracic glands (PG) of Bombyx mori in its larva-to-larva development. The timing of PTTH release was estimated by examining developmental response of 4th instar larvae to brain removal and neck ligation, and changes in the haemolymph ecdysteroid titer and ecdysone-releasing activity of PG in vitro during the development. Injection of Bombyx brain extracts into 4th instar larvae neck-ligated shortly before full activation of PG elicited larval moulting rather than precocious pupation in headless larvae. This developmental shift was regarded as due to the action of PTTH, and the PTTH unit has been defined from a linear log dose-response relationship. Materials chromatographically fractionated from Bombyx brain extracts were examined for the presence of stage- and species-specific PTTH molecules by using this Bombyx larva assay and Bombyx and Samia pupa assays previously developed. The same fra...

Foot-and-mouth disease vaccination: a multifactorial study of the influence of antigen dose and potentially competitive immunogens on the response of cattle of different ages

Groups of 68 and 66 cattle aged 12 and 24 months respectively were each subdivided into 16 groups and inoculated with foot-and-mouth disease vaccines containing O1 Campos, A24 Cruzeiro and C3 Pando virus strains. The 140S antigen mass of the O1 and A24 valencies was varied while that of C3 was held constant. Multifactorial comparisons between the 21 day serum neutralising antibody titres showed that over most of the range there was a linear log dose response relationship. Doubling the antigen dose increased the serum antibody titres against both A24 Cruzeiro and O1 Campos by approximately 0.15 log10. The A24 antigen was about 30 times more immunogenic than the O1 with C3 intermediate between the two. At high antigen doses the responses flattened but the level at which this occurred depended on the immunogen administered. No difference could be demonstrated between the responses of 12- and 24-month-old cattle and there was no evidence of competitive inhibition or enhancement between the virus strains included in the vaccines.