Three mouse tumor lines, normally growing progressively when grafted intradermally into syngeneic mice, failed to grow when injected as a mixture with Bacillus Calmette-Guérin (BCG) into mice immune to BCG. Experiments to investigate the processes involved in this tumor suppression at the site of BCG injection led to the following conclusions: 1) The effect of the BCG is local, not systemic. 2) Lack of tumor growth cannot be accounted for by death of tumor cells in the BCG mixture before injection. 3) There is evidence that tumor growth is slightly retarded in animals unable to respond immunologically to either BCG or tumor antigens; however, this apparently nonimmunological process is not sufficient to account for the complete suppression of tumor growth observed. 4) Host immune reactivity to BCG is necessary to prevent growth of viable tumor cells injected with the BCG. 5) Tumor growth at the site of host reaction to BCG may be suppressed in the absence of tumor-specific immunity. 6) Established, growing tumors may regress if infiltrated with BCG. 7) Combination of tumor infiltration with BCG and of subsequent surgery is more likely to be curative than either modality alone. 8) With the tumor most thoroughly studied, injection of a mixture of tumor and BCG does not induce tumor immunity as well as does temporary growth of the tumor alone.