The antitumor activity of a preparation of heat-killed cells of Enterococcus faecalis, FK-23. Intraperitoneal injection of the preparation prolonged the lifespan of C3H/He N mice which were intraperitoneally inoculated with MM46 mammary carcinoma. Not only intraperitoneal but also oral administration of the FK-23 preparation inhibited the growth of these carcinomas inoculated intradermally. The tumor-bearing mice produced tumor necrosis factor (TNF) in their sera 2 h after intravenous injection of OK432. This TNF level increased after the mice were fed with food supplemented with the FK-23 preparation. Additionally, the FK-23 preparation inhibited the growth of Meth A fibrosarcoma in cyclophosphamide-treated BALB/c mice.