Svobodova Z., L. Groch, M. Flajshans, B. Vykusova. 1. Machova: The Effect of Long-term Therapeutic Bath of Malachite Gteen on Common Carp (C-,prinus carpio L.). Acta vet. Bmo 1997,66: 111-117. The aim of this paper was to determine acute toxicity of malachite green of the Vert Malachite BE SA Crystaux type for common carp and to evaluate the effect of a 6-day therapeutic bath in O.S mg.!-I concentration of this pharmaceutical on values of selected haematological indicators including micronuclei test, as well as on histological composition of tissues of oneto two-year-old carp. The 48hLCSO value of the malachite green tested was 0.84 mg.!-I. After the 6-day bath, changes in red blood picture [decrease in haematocrit value PCV (P<O.OOI), decrease in mean corpuscular volume MCV (P<O.OS) and increase in mean corpuscular haemoglobin concentration MCHC (p<0.0 I) 1 as well as in white blood picture /decrease in both relative and absolute monocyte count (P<O.OS and P<O.OI), respectively/ were recorded in treated fish (n=30) compared to the control group (n=29). Other indices of red and white blood picture in these fish did not differ from the control group. A decrease in total protein concentration (TP, P<O.O I) in blood plasma was found in fish treated with malachite green. By means of a micronuclei test, no effect of long-term bath in malachite green on increase of their frequency in erythrocytes was proved. Histological examination revealed mild regressive changes on gills, mild dystrophic changes in parenchymatous tissues and an increased macrophage activity in treated fish. These changes indicate an increased sensitivity of the fish treated with malachite green to hypoxia and impaired proteosynthesis. Acute toxicity, haematology, micronuclei test, histopathological changes Malachite green is stilI one of the mostly used preparations for inhibition of protozoal ectoparasitoses and mycoses of fish and their eggs (N 0 g a 1995), although it has numerous negative side effects. Its current importance even increased, since it was recommended for therapy ofa proliferative kidney disease PKD (C I i fton H ad ley and AI derm an 1987, 1988; Deb u f 1991). The negative effects of malachite green are as follows: high acute toxicity for fish (Machova and Svobodova 1984; Alderman 1985; Obiekezic and 0 k a for 1995; Van H e e r den et ai. 1995), capacity to accumulate in fish and longterm persistence of residues in tissues (B au e r et ai. 1988; A Ide r man 1988; A Ide r man and Clifton-Hadley 1993: Machova et al. 1996). Furthermore, malachite green is characterized as respiratory poison, teratogen and suspect carcinogen (M eye rand J 0 r g ens e n 1983). The negative effect of therapeutic baths in malachite green on haematological indicators and histopathological status of tissues was reported by numerous authorsinsalmonids(Ross etaI.1985; Pickering and Pottinger 1985; Hlavek and Bulkley 1980; Gerundo etaI.I991)andinsilurids(Srivastava etaI.1995). This paper deals with acute toxicity determination of the malachite green Vert Malachite 13 ESA Crystaux (imported from Italy) and with observation on the effect of a long-term therapeutic bath in this pharmaceutical on haematological indicators and histopathological composition of tissues of common carp. This type of malachite green was succesfully introduced and distributed among Czech fish farms.