The injection of antiserum against isogeneic aggregated mouse immunoglobulins prolonged the survival time of skin allografts in mice by 5-6 days. The study of the possible mechanisms of this phenomenon revealed that the injection of aggregated immunoglobulin antiserum decreased the ability of the lymphocytes of the animals receiving the antiserum to induce local "graft versus host" reaction and to proliferate in the blast transformation test in response to phytohemagglutinin treatment and in the mixed lymphocyte culture test. When added in vitro, the antiserum did not influence either the mixed lymphocyte, or the specific cytotoxic effect of immune lymphocytes, but stimulated 3H-thymidine incorporation into the suspension of lymphocytes incubated with syngeneic irradiated cells. The latter fact may be indicative of the mitogenic action of the antiserum. The analogy between the mitogenic action of the antiserum and that of nonspecific mitogens, such as concanavalin A and phytohemagglutinin which also show opposite effects in vivo and in vitro, is discussed. The conclusion has been made that the increase of the survival time of the skin allograft after the multiple injection of aggregated immunoglobulin antiserum was primarily due to its action on the recognizing and proliferative functions of T lymphocytes, and later due to an increase in IgG synthesis.