We had demonstrated that the NK cell mediated cytotoxicity of murine spleen cells could be augmented by in vivo priming and subsequent in vitro challenge with a streptococcal preparation OK432, and the cell surface phenotype of induced killer cells was Thy-1+, asialo GM1+, suggesting that the activated cells were of NK lineage (OK-NK cell). We had also clarified that IL-2 played a major role in inducing the OK-NK cells via the production of IFN-gamma. In this study, we examined the effect of adoptive transfer of OK-NK cells on syngeneic tumors in mice. Mice were implanted with SP2 myeloma cells intraperitoneally (i.p.), or C26 colon adenocarcinoma cells subcutaneously to make the models of peritonitis carcinomatosa or solid tumor, and the OK-NK cells were transferred i.p. or intratumorally, adoptively. By the adoptive transfer of OK-NK cells, 92% of mice bearing SP2-tumor had be cured. The tumor growth of C26-solid tumor was inhibited, and the survival rate of mice bearing C26-tumor was significantly increased. The intratumoral remnants of 125I-labelled OK-NK cells were 61, 27 and 8% at 4, 12 and 36h after intratumoral transfer, respectively. By multiple transfer of OK-NK cells, the antitumor effect was more effectively augmented than that of a single transfer. Results in this study suggested that OK-NK cells could be useful for the therapy of cancer patients.