The goal of this paper was to test the time of malachite green persist
ence in rainbow trout after long-term bath as well as to evaluate the
quality of randomly sampled marketable rainbow trout from the point of
view of malachite green residues. A 6-day-lasting therapeutic bath of
rainbow trout was performed in malachite green of 0.2 mg.l(-1) concen
tration. Its residues were then observed in muscle, liver and skin of
treated fish. Immediately after the bath, malachite green was detected
in muscle, liver and skin at the levels of 0.712 +/- 0.383, 0.834 and
0.649 mg.kg(-1), respectively, as a sum of both coloured form and leu
koform. Eight weeks after the bath, an expressive decrease of the colo
ured form of malachite green was found while its leukoform was detecte
d in fish as late as after 10 months. Results of analyses were found n
egative twelve months after the bath. Marketable rainbow trout were ra
ndomly sampled for checking the malachite green content on 12 trout fa
rms in the Czech Republic in 1993-1995. Malachite green was found in 2
case (in 1 of them only as a slightly increased value in skin) while
all other findings were negative. Hence, the results of our study indi
cate that malachite green persisted in the treated fish as a leukoform
for 10 months. It was evident that the 6-months withdrawal time, reco
mmended in the Czech Republic was insufficient. In case of the frequen
t one-and-half-year-long productive cycle of rainbow trout we therefor
e do not recommend to carry out therapeutic baths in malachite green f
or fish older than 6 months.