We showed previously that T cells with the phenotype Leu-3+,8+ are required for the induction of antigen-specific Leu-2+ suppressor cells. Furthermore, when mixed lymphocyte reactions are carried out in the presence of 1 microgram/ml cyclosporin A (CsA), such cultures lead preferentially to the activation of alloantigen-specific suppressor-inducer Leu-3+,8+ cells. In an attempt to generate a clone of T cells with such specific suppressor-inducer properties, we activated Leu-3+,8+ T cells with allogeneic (HLA-DR4+) lymphocytes in the presence of CsA. Clone SP-21, derived by propagating such activated T cells with conditioned medium containing IL 2, is a noncytotoxic, nonsuppressor clone that specifically proliferates to allogeneic cells bearing HLA-DR4 antigen. When cultured with fresh autologous Leu-2+ cells in the absence of HLA-DR4+ cells, clone SP-21 selectively activates Leu-2+ suppressor cells, which inhibit the response of fresh Leu-3+ cells to DR4+ stimulator cells. On the other hand, clone SP-21 fails to induce cytolytic T cells or to help B cell differentiation. These results demonstrate that a T cell clone with a remarkably narrow functional repertoire nonetheless contains and transmits all of the signals necessary for the activation of antigen-specific suppressor cells.