AbstractThe therapeutic effect of endogenous tumor necrosis factor (TNF) on Meth A ascites fibrosarcoma in mice was investigated. Serum and peritoneal fluid from tumor bearing mice treated with OK-432 and LPS were cytotoxic to tumor cells in vitro. The peak of cytotoxicity in both the serum and peritoneal fluid was found in the fraction corresponding to a molecular weight of approximately 54,000–56,000 on HPLC and the PI was found to be 4.9–5.1 by is oelectric focusing. These results are consistent with previously reported findings on TNF, and indicate that endogenous TNF has a satisfactory life-prolonging effect.The tumor necrosis factor (TNF) is considered to be one of the clinically most promising anti-cancer cytokines because of its potent and very specific antitumor effect on target cells (Carswell, Old, Kassel, Green, Fiore&Williamson, 1975; Matthews&Watkins, 1978; Niitsu, Watanabe&Urushizaki, 1984).TNF as an anti-cancer cytokine for the treatment of cancer may be applied in one of the two following ways: 1) by administration of purified TNF or 2) by endogenously inducing TNF in cancer bearing individuals. The antitumor effects of TNF administered exogenously have been examined using crude preparations or serum containings TMF (tumor necrosis serun, TNS) (Carswell et al., 1975; Watanabe, Niitsu, Sone, Neda, Ishigaki&Urushizaki, 1984).In a previous paper we reported that mice primed with OK-432 and challenged with endotoxin produced a soluble cytotoxic factor in peritoneal fluids (Yamamoto, Nagamuta, Usami, Sugawara, Watanabe, Niitsu&Urushizaki, 1985; Nagamuta, Yamamoto, Usami, Sugawara, Watanabe, Niitsu&Urushizaki, 1985).This peritoneal cytotoxic factor (PCF) had cytostatic and/or cytotoxic effect not only on mouse tumor cell lines but also on human tumor cell lines without species specificity. Normal cell lines were not affected. Here we report the endogenous production of TNF in tumor bearing mice and its antitumor effects.