C3H mice were subcutaneously (s.c.) injected with a tumorigenic dose (1mg/mouse) of 3-methylcholanthrene (MC) and tumor development was observed for 18 weeks. OK-432 or Ge-132 was intraperitoneally administered either after MC injection (post-treatment) or both before and after MC injection (pre- and post-treatment). Pre- and post-treatment with OK-432 significantly inhibited the development of tumors from 10 to 12 weeks after MC injection as compared with a control group. In all of the groups, the incidence of tumors was 100 percent 18 weeks after MC injection. With respect to Ge-132, both post-treatment and pre- and post-treatment administration reduced tumor growth in a similar manner to pre- and post-treatment of OK-432. After pre-treatment with OK-432 or Ge-132 for two weeks, the percentage of large granular lymphocytes in peripheral blood and the natural killer activity of splenic cells in mice were increased. These results suggest that the immune status plays an important role in the defense mechanisms against chemically-induced tumors.