The number of leucocytes and thrombocytes in the circulating blood of fishes seems to reflect more accurately than the number of erythrocytes the fish's reaction to acute stress, including that caused by pollution. To facilitate the estimation of leucocytes in fish blood, a new test is being introduced called leucocrit, which is based on the determination of the volume of packed leucocytes–thrombocytes expressed as a percentage of the whole blood. For stocks of juvenile coho salmon (Oncorhynchus kisutch) and rainbow trout (Salmo gairdneri) held in freshwater under a variety of conditions, mean leucocrit values varied from 0.55 to 1.91%. Variations in age, species, food ration, degree of crowding, and prior disturbances offish stocks were associated with these differences. Leucocrit and leucocyte–thrombocyte counts for both species were depressed from stock values after 96-h or shorter (rainbow) exposure to stressful (high-temperature crowded) conditions; values for these measures were positively correlated but uncorrected with hematocrit values. Hematological values for subpopulations of underyearling rainbow trout reared under summer or winter photoperiod conditions were unaffected by photoperiod and responded similarly to acute stress. Leucocrit values for stocks of coho (1.30 ± 0.27%) and rainbow (0.92 ± 0.12%) were depressed due to transfer or acute exposure to bleached kraft pulpmill effluent (BKME). A 24-h exposure to BKME after 2 days' acclimation caused consistent leucocrit depressions, with median effective concentrations of 0.28 of the 96-h LC50 value for coho and 0.16 LC50 for rainbow. Hematocrit showed less sensitive or consistent changes due to stress or effluent exposure than leucocrit. Key words: leucocrit, leucocyte–thrombocyte count, hematocrit, blood, acute stress, BKME, pulpmill effluent, sublethal, salmonid fish, applications

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