Stimulation of mixed lymphocyte cultures with ultraviolet (UV)-irradiated stimulator cells leads to selective activation of Lyt-2+ cytotoxic T lymphocyte precursors (CTL-P) to acquire interleukin 2 (IL2) reactivity: (a) no proliferative response, nor IL2 production was observed in these cultures; (b) upon addition of preformed IL2-conditioned media (IL2-CM), a vigorous proliferative response was observed; (c) the IL2-dependent proliferation was reduced by approximately 90% after depletion of Lyt-2+ responder cells; and (d) specific CTL were generated upon the initial triggering by UV-irradiated stimulators and IL2 expansion. This system therefore provides suitable conditions for studying the role of Lyt-2 antigens in initial, antigen-specific induction of CTL-P, as well as at their effector phase. It is shown herein that monoclonal anti-Lyt-2 antibodies inhibit antigen-specific acquisition of IL2 reactivity by CTL-P, which results in 85% reduction of the proliferative response mediated by IL2-CM. CTL-P which have been induced by antigen to IL2 reactivity, prior to addition of anti-Lyt-2 antibodies, however, are not affected in their IL2-dependent proliferation. Furthermore, both the specific and lectin-dependent cytolytic activity of effector CTL generated by UV-irradiated stimulators and IL2-CM is blocked by addition of anti-Lyt-2 antibodies in the cytotoxicity assay. The implications of these findings for the role of Lyt-2 antigen in antigen recognition by T cells are discussed.