Abstract The antitumoral effect of BCG has been compared to the antitumoral effect of injections of irradiated isogeneic leukemic cells in a transplantable, murine ascites leukemia. It was found that irradiated tumor cells in doses of 10 7 or 10 9 cells per injection (either s.c. or i.p.) induced prolongation of mean survival time and increased the percentage of cured mice. The effect of irradiated cells was highest with s.c. injections and an effect was observed only in mice inoculated with 10 2 or 10 3 living leukemic cells. BCG had only beneficial effect when given i.v. and only in a dose of 7 × 10 6 viable units per injection. Lower doses of BCG had no effect compared to controls and higher doses given i.v. or i.p. enhanced tumor growth. BCG injected i.v. may induce a cure in mice inoculated with 10 2−10 5 living leukemic cells. Spontaneously cured mice were highly susceptible to a second tumor inoculum containing a tumor cell number only one decade higher than the first inoculum. In contrast, immunostimulated mice rejected inocula 2–3 decades above first inoculum. Histologic examination of organs from mice cured either spontaneously or by immunostimulation revealed that BCG induced markedly histiocytic proliferation and infiltration in spleen, thymus, lymph nodes, liver, and lungs, whereas otherwise cured mice had no such lesions.