Reactive lymphocyte blastogenesis was investigated by 3H-thymidine uptake of peripheral lymphocytes. Canine renal allotransplantations were performed in 19 dogs, which were divided into three groups by different immunosuppressive therapies. The first group, not treated with any immunosuppressants, showed the highest 3H-thymidine uptake, but the third group, treated with prednisolone (1.5 mg/kg/day) and azathioprine (4 mg/kg/day), did not show remarkable 3H-thymidine uptake except for 2 dogs which died of severe acute rejection. Single use of prednisolone (second group) was less effective in suppression of blastogenesis compared with the combined use of azathioprine and prednisolone. In addition to these experiments, the effects of surgical operation, blood transfusion, and infection on lymphocyte blastogenesis were investigated. Renal autotransplantation did not produce remarkable blastogenesis, but blood transfusion and infection caused strong lymphocyte blastogenesis. Periodical increases of 3H-thymidine uptake were observed in the infected animal.