A study was made on a mode of action of the streptococcal preparation, OK-432, that is rather difficult to elucidate only in terms of its direct effect on tumor cells. Its results are described below: 1) The survival rate of the ICR mice pretreated i.p. with OK-432 was about twice as high as in the nontreated group at 40 days after i.v. inoculation of Ehrlich carcinoma. 2) The survival rate of the C3H mice pretreated i.p. with OK-432 was more prolonged than the rate of nontreated group and post-treated group with OK-432 at 50 days after i.p. inoculation of methylcholanthrene-induced sarcoma. 3) The survival rate of the ICR mice pretreated s.c. with OK-432 was slightly prolonged than the rate of nontreated group at 40 days after i.p. inoculation of Ehrlich carcinoma. 4) The appearance of methylcholanthrene-induced sarcoma of the C3H mice pretreated i.p. with OK-432 was significantly delayed, compared with the nontreated group and the posttreated group at 80 days after injection s.c. with methylcholanthrene. The results stated above lead to the conclusion that OK-432 has a host-mediated antitumor action.