The antitumor activity of the cell-wall skeleton (CWS) of Propionibacterium acnes C7 was examined by using transplantable tumors in syngeneic mice and in guinea pigs, and autochthonous tumors in mice. P. acnes-CWS was shown to suppress the growth of fibrosarcomas, EL4 leukemia, and MH134 hepatoma in syngeneic mice, and to regress the established tumors of a fibrosarcoma (MC104) in C57BL/6J mice, and a hepatoma (line-10) in strain-2 guinea pigs. The oil-attached P. acnes-CWS mixed with fructose mycolate was effective for suppression of the autograft of fibrosarcoma in mice. The repeated intralesional injections of suspension of P. acnes-CWS in phosphate-buffered saline was effective for prolongation of survival period of mice bearing 3-methylcholanthrene-induced fibrosarcoma. The test results on the cell fractions of P. acnes indicated that the CWS, but not the cytoplasmic or glucan fraction, of P. acnes had anti-tumor activity. The activation of peritoneal macrophages of mice was observed when P. acnes-CWS, but not the cytoplasmic fraction, was injected intraperitoneally 4 days before. The relationship between the cytolytic activity of peritoneal macrophages and antitumor activities of P. acnes-CWS was also discussed.