Abstract: The aim of this study was to determine the effect of testosterone(T)on immune activities of lymphocytes in rainbow trout, Oncorhynchus mykiss. Plasma IgM level deceased, when plasma T level was elevated by in vivo treatment of T. Moreover, the number of antibody-producing cells and the amount of 1gM in the culture media also deceased in a dosage-dependent manner in rivo under different concentrations of T. It is revealed that T can suppress lymphocyte activities, at least the secretion of 1gM from B lymphocytes, suggesting that rainbow trout susceptibility to infectious diseases is closely related to the elevation of T level during spawning season.