The aim of this study was to determine the percentage of individual types o
f leukocytes in fish of economic (carp - Cyprinus carpio L., tench Tinca ti
nca L., European catfish - Silurus glanis L., rainbow trout - Oncorhynchus
mykiss Walb.,) and indicator importance (bream - Abramis brama L., perch -
Perca fluviatilis L., chub - Leuciscus cephalus L., brown trout - Salmo tru
tta m. fario L.). In all, we examined 191 fish specimens caught in non-cont
aminated or insignificantly contaminated localities in good state of health
during the vegetation season and not within the season of their reproducti
on. Leukocytes stained according to Giemsa were classified as: lymphocytes,
monocytes, myelocytes, metamyelocytes, neutrophilic granulocytes with band
and segmented nuclei, eosinophils and basophils. Blood of all the fish spe
cies examined was of lymphocytic character. The mean percentage of individu
al lymphocytes varied from 84.2 +/- 5.08 to 99.1 +/- 0.99%. The lowest perc
entage of lymphocytes was found in the rainbow and brown trout (84.2 +/- 5.
08 and 87.3 +/- 7.21%, respectively), while the highest values were in the
perch (99.1 +/- 0.99 and 97.5 +/- 3.19%). Monocytes were sporadic in all th
e fish species; with the highest mean value found in the female spawner Eur
opean catfish (1.42 +/- 1.74%. The highest percentage of granulocytic serie
s cells was found in rainbow (15.4 +/- 7.80%) and brown trout (11.6 +/- 7.2
3%); whereas the lowest values in the perch (0.8 +/- 1.01 and 2.4 +/- 3.15%
). As far as the granulocytic series cells are concerned, metamyelocytes we
re most common with regards to their percentage in all the fish species exc
epting salmonids. There were prevailing band and segmented neutrophils in t
he rainbow and brown trout. Granulocytes were mostly represented by neutrop
hils in all the fish species studied. Eosinophilic granulocytes were found
sporadically in the tench (0.17 +/- 0.59%) and in 3-year old carp (0.10 +/-
0.19%). Basophilic granulocytes were not found in any of the fish species
examined. We found practically equal percentage of individual types of leuk
ocytes in the carp, bream and perch of various age categories caught under
the same conditions and in the same season of the year. There were no signi
ficant differences in the percentage of individual leukocytes in male and f
emale spawners of 3-year-old carp, 3-year-old tench, and 2- to 3-year-old b
rown trout. Our results contribute to the use and help in introducing the e
xamination of differential leukocyte counts as a biological monitoring meth
od to evaluate surface water contamination.