In the murine syngeneic mixed lymphocyte response (SMLR), the co-culture of nylon wool-nonadherent spleen cells or young (4 to 6-day-old) thymocytes with syngeneic nylon wool-adherent adult spleen cells, results in proliferation and Interleukin 2 (IL 2) production. In both the thymus and spleen SMLR, proliferation and IL 2 production are the function of Lyt-1+2-3- (Lyt-1) responder cells and require the expression of IA (IB) and/or IE cell surface antigens on the stimulator cell. IL 2 production in both spleen and thymus SMLR is independent of heterologous serum factors (i.e., fetal calf serum), which, however, influences the magnitude of the in vitro proliferation. The IL 2-producing cell in spleen is capable of self-renewal in the absence of the thymus; it is detectable 6 to 7 mo after adult thymectomy. These results suggest the in vitro SMLR may measure a cellular interaction between the Lyt-1 subset of T cells and self Ia antigens on macrophage-like cells that produces IL 2 in an antigen-independent fashion. This interaction may have physiologic significance in the expression and regulation of T cell function.