It is well established that immune potential declines with age and that aged animals are unable to mount vigorous cell-mediated immune responses. Production of the lymphokine, Interleukin 2 (IL 2) in response to the T cell mitogen concanavalin A or allogeneic stimulation is also diminished in aged individuals. This lesion in IL 2 synthesis in aged mice underlies their deficiency in cell-mediated responses. Mixed lymphocyte responses, cell-mediated lympholysis, and antigen-induced T cell proliferation are all markedly enhanced by supplementation with this lymphokine.