The responsiveness of murine thymocytes to T cell mitogens was examined in the presence of highly purified preparations of IL 1 and IL 2. Fractionation of thymocytes by selective agglutination with peanut lectin (Arachius hypogeae) resulted in separation of two cell populations, each of which mediated distinctive responses to cytokine stimulation. The majority of thymus cells, agglutinated by the lectin (PNA+), were found to be unresponsive to mitogen in the presence of the macrophage-derived factor, IL 1; but showed marked proliferation when stimulated by mitogen in concert with the T cell-derived factor, IL 2. The nonagglutinated fraction, (PNA- cells) demonstrated substantial levels of proliferation after stimulation with mitogen in the absence of any exogenous factors. Moreover, IL 1 alone was highly mitogenic for PNA-thymocytes. This stimulation was further shown to result in the subsequent release of IL 2, as assessed by both the detection of IL 2 by lymphokine-dependent T cell lines and the inhibition of IL 1-dependent mitogenesis by the presence of anti-IL 2 monoclonal antibody. These results further substantiate the important immunoregulatory nature of these proteins and elucidates the possible role of these substances in the ontogeny of T cell responsiveness.