T cells cultured at limit dilution for 8 days in a concanavalin A-stimulated, filler-cell and growth factor-supported system produced cytolytic clones with high efficiency. These clones were not specific, lysing a wide range of targets, syngeneic and allogeneic, of tumor and normal cell origin. Lysis was a cell-mediated phenomenon but was not blocked by anti-Ly-2. One H-2-negative target was lysed, but one was resistant. Xenogeneic (human) tumor cells were not lysed. The cells in the clones were large, vacuolated, granular lymphocytes. They originated from single Ly-2+ responder cells and not from irradiated filler cells. Therefore, activated lymphocyte killers and other natural killer-like cells may be differentiated elements of the Ly-2+ T-cell lineage.