The incorporation of 3H-thymidine into the acid insoluble fraction of spleen cells was increased intraperitoneal injection of 10 microgram bestatin/mouse. Bestatin (0.01, 0.1, 1.0 microgram/ml) which was added to mouse spleen cell culture increased 3H-thymidine incorporation into lymphocytes, but 10 microgram/ml did not. This mitogenic action of bestatin was not observed when adherent cells were removed or T cells were destroyed, suggesting that bestatin causes the proliferate T cells probably through the activation of macrophages. Bestatin given to mice did not modulate mitogenicity of lectins. Bestatin treatment of human peripheral buffy coat cells increased 3H-thymidine incorporation into lymphocytes. The addition of bestatin at a high concentration such as 100 microgram/ml to mouse spleen cell cultures exhibited a high mitogenic effect on B cells in preference to T cells but this effect was not seen with bestatin at 50 microgram/ml. In this case, the mitogenicity of Con A or LPS was expanded and antibody formation to SRBC in spleen cell cultures were also stimulated.