The intradermal inoculation of mixtures containing living tumor cells and living Mycobacterium bovis (strain BCG) into unimmunized syngeneic guinea pigs results in an inflammatory reaction to the BCG, and there is no progressive tumor growth. In the absence of BCG the tumor grows progressively, metastasizes, and kills the animal. By conventional methods, it has not been possible to immunize syngeneic guinea pigs to the tumor used. Guinea pigs that receive mixtures of BCG and tumor cells, however, develop specific systemic tumor immunity as measured by delayed cutaneous hypersensitivity and by suppression of tumor growth.