Human peripheral blood mononuclear (PBM) cells incubated for 3 or more days in medium containing streptococcal preparation OK-432 developed toxicity to natural killer-resistant tumor cell lines. The generation of cytotoxic cells by OK-432 was potentiated by exogenous lymphokines such as interleukin 2 (IL-2) and interferon-gamma (IFN-gamma), the former being much more effective than the latter, and was blocked by antibodies to these cytokines such as interleukin 1 (IL-1), IL-2, and IFN-gamma. Although monocyte-depleted PBM cells were barely responsive to OK-432, they gave rise to activated killer (AK) cells after being incubated in medium with both IL-2 and IFN-gamma. Their generation was augmented by exogenous IL-1 but not by OK-432. These data indicate that AK-like cells are generated from human PBM cells through mediation of IL-1, IL-2, and IFN-gamma produced by OK-432 in vitro.