The incorporation of 3H-thymidine into the acid insoluble fraction of spleen cells was increased by i.p. injection of 10 .mu.g bestatin/mouse. Bestatin (0.01, 0.1, 1.0 .mu.g/ml), added to mouse spleen cell culture, increased 3H-thymidine incorporation into lymphocyte 10 .mu.g/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 proliferation of T cells probably through the activation of macrophages. Bestatin given to mice did not modulate mitogenicity of lectins [concanavalin (Con) A, phytohemagglutinin, pokeweed mitogen, lipopolysaccharide (LPS)]. 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 .mu.g/ml to mouse spleen cell cultures exhibited a high mitogenic effect on B cells in preference to T cells, this effect was not seen with 50 .mu.g/ml bestatin. The mitogenicity of Con A or LPS was expanded and antibody formation to SRBC [sheep red blood cells] in spleen cell cultures was stimulated.