Sensitization with mitomycin C-treated L1210 or EL-4 tumor cells followed by intraperitoneal injection of a streptococcal preparation OK-432 rendered histocompatible or syngeneic mice immune to the corresponding tumor cells. The antitumor immunity, which was more potent than that induced by attenuated tumor cells alone, was manifested by transplantation resistance to challenge tumor cells, and by cytotoxic activity of spleen cells from the primed mice. The former activity was closely related to the latter, which was found to be mainly due to tumor-specific cytotoxic T lymphocytes. The in vivo immunoaugmentation by OK-432 was susceptible to macrophage toxins such as trypan blue and carragheenins, and was partly dependent on the activity of noncytotoxic Ia-positive peritoneal macrophages. OK-432-mediated enhancement of Ia-positive macrophage functions was confirmed by concanavalin A-blastogenesis and T cell-dependent antibody formation. Allo-reactive cytotoxicity induced in allogeneic or semiallogeneic mice, which had been primed with clonogenic or attenuated tumor cells, was also augmented by concomitant administration of OK-432. These results suggest that OK-432 augments induction of antitumor immunity and alloreactive cytotoxicity, associated with stimulation of noncytotoxic Ia-positive accessory macrophage activity.