When peripheral blood lymphocytes from healthy adults are cultured with autologous (auto) or allogeneic (allo) Epstein-Barr virus-transformed cells (LCL), non-specific killer activity against NK-sensitive K562 and NK-resistant Raji, as well as specific killer activity against LCL is enhanced or generated. We analyzed the cell subsets possessing such cytotoxicity using monoclonal antibodies (MoAb). OKT3, a MoAb to T cell receptor-associated molecule, added in the effector phase suppressed the killer activity against LCL but not against Raji or K562. In contrast, OKT3 added in the induction phase abolished the generation of cytotoxicity against all targets. The addition of OKT8 in either the effector or induction phase inhibited anti-LCL killing induced by stimulation with alloLCL. This suggests that CD8 is required for recognition of alloLCL. The treatment of effector cells with MoAb and complement(C) revealed that killers against LCL were OKT8+ Leu11-, and those against K562 were OKT8- Leu11+. When auto-LCL were used as stimulator, removal of OKT4+ cells in the induction phase diminished the cytotoxicity against all targets, indicating that CD4+ T cells recognize autoLCL. Elimination of CD8+ cells from responder did not decrease the generation of killer activity. Further experiments suggested that this was caused by the coexistence of CD4+ killer cells or by the increase of residual CD8+ effector cells.