Plasma γ-globulin fraction proteins, copper and zinc cations, and metal complexes formed by these cations and human serum γ-globulin induce blast transformation of splenocytes from BALB/c mice at a level comparable to that induced by concanavalin A. Zinc bound to γ-globulin reduces by 25% and copper in complex with this protein stimulates by 1.6 times its capacity to induce blast transformation. Combinations with concanavalin A reproduce the effects of γ-globulin-metal complex under conditions of mitogen induction. Incorporation of(3)H-thymidine in splenocytes incubated with combinations of γ-globulin-copper metalcomplex, copper cations, and control protein with concanavalin A was by 1.4, 1.3 (p<0.1), and 1.25 times higher (p<0.05), respectively, than after incubation with concanavalin A alone.