Fresh living Bacillus Calmette-Guerin (BeG), injected i.v. into (C57BI/ 6xDBA/2)FI mice, activated peritoneal macrophages rendering them highly cytotoxic for tumor cells in vitro. This cytotoxic activity w s already maximal 14 days after injection of 1 mg of BCG and remained stable when 3 or 5 mg of BCG were given. At the same time spleen cells of the BCG-treat d mice showed strongly depressed responses to the T-cell mitogens, PHA and Con A, irrespective of the dose of BCG injected. The inhibitory effect was shown to be mediated by suppressor cells which had characteristics of macrophages since they could be removed by carbonyl iron and magnet treatment and were adherent to plastic. In contrast to it was observed after injec- tion of 1 mg of BCG, these suppressor cells alone did not account for the depression of T-cell re.sponses induced by higher doses of BCG. Nylon-nonadherent cell populations obtained from spleen cells treated with 3 or 5 mg BCG partially retained the inhibitory activity suggesting that suppressor T cells were also induced after injection of high doses of BCG. In contrast, the responses to the B-cell mitogen LPS of unfractionated and macrophage-depleted spleen cells were not affected or significantly enhanced depending on the dose of BCG injected.