The antitumor activity of Bifidobacterium breve K-110, and K-I11, and B. infantis K-525 was investigated. These Bifidobacterial cells and their cell wail preparations (WPG) significantly increased the survival rate of mice who had been intraperitoneally implanted with sarcoma 180 cells. Solid tumor growth was inhibited even when the sarcoma 180 cells were implanted into the groins of the mice. However, the Bifidobacterial cells did not show in vitro cytotoxicity against tumor cell lines. Cell kinetic studies revealed that these WPGs induced neutrophils, which were followed by macrophages, at the site of peritoneal injection. The WPGs directly activated these cells to inhibit the growth of tumor cells in vitro assays. Our results suggest that Bifidobacterial WPGs induce and activate nonspecific phagocytes in situ to reject growing tumor cells in the mouse peritoneal cavity.