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The temperature dependence of in vivo activation of rainbow trout Oncorhynchus mykiss, leucocyte populations after intraperitoneal injection (i.p.) of fish with a T-cell independent antigen Aeromonas salmonicida (strain MT423) was investigated using a proliferation assay and flow cytometric analysis with mab specific for trout leucocyte surface markers.In trout kept at 15–17° C a prominent activation of blood and spleen leucocytes was found. Also, drastic changes of the percentage of the leucocyte populations in blood and spleen occurred: the amount of monocytes in the blood increased between day 2 and day 7 post injection (p.i.), whereas in spleen the amount of monocytes stayed at a high level (∼35%) after a depression between day 4 and day 7 p.i. The percentage of B-lymphocytes was increased first in spleen and then in blood. The percentage of granulocytes in blood was elevated during the whole experiment compared to control fish.In trout kept at 10–12° C only blood leucocytes showed a weak activation after i.p. injection of A. salmonicida, whereas spleen leucocytes showed nearly no reaction. Only the percentage of granulocytes in the blood (day 2–14 p.i.) and of monocytes in the spleen (day 2 and day 8 p.i.) was changed compared to phosphate buffered saline (PBS)-injected fish.However, the development of A. salmonicida specific antibodies was contrary to the cellular reaction. Whereas antibodies could first be detected after 16–18 days p.i. in both groups the amount of antibodies was significantly higher in sera of trout kept at 10–12° C at day 22 and day 28 p.i. than in sera of trout kept at 15–17° C.These results indicate stronger A. salmonicida induced activation of monocytes, granulocytes and B-lymphocytes at higher temperature. However, the development of a specific antibody response against A. salmonicida seemed to be more effective at lower temperatures. [Copyright &y& Elsevier]