The optimal concentration of interleukin 2 (IL 2) for maintaining the in vitro growth of T cells was quite different from that required for the induction of cytotoxic T lymphocytes (CTL) in nu/nu spleen cells. Higher concentrations of IL 2-containing preparations (10 to 30% v/v or 5 to 15 U/ml) were needed to promote the T cell growth, whereas lower concentrations (1 to 3% v/v or 0.5 to 1.5 U/ml) were needed to generate alloreactive CTL. It was further shown that the addition of high concentrations of IL 2 (10 to 30%) suppressed the generation of alloreactive CTL in conventional MLC. High concentrations of IL 2 induced the generation of antigen-nonspecific suppressor T cells in normal spleen cell cultures and augmented the generation of antigen-specific suppressor T cells in MLC. These suppressor cells suppressed the generation of CTL in fresh MLC and in polyclonal CTL cultures. These suppressor T cells could be induced by rat (spleen)-produced, murine (EL-4 cells) produced IL 2 preparations, and a purified human recombinant IL 2 (HR-IL2). The ability to induce suppressor cells correlated with the activity of IL 2 present in these preparations and was independent of their ability to induce cytotoxic effectors. These findings indicate that IL 2 may play a dual role in the regulation of CTL responses. We suggest that during antigen sensitization, the initial endogenous production of lower levels of IL 2 provided the second signal for the differentiation and proliferation of CTL. When higher levels of IL 2 were produced later, the suppressor T cell precursors were activated and differentiated into suppressor effectors to regulate the CTL response.