We earlier demonstrated that 50% of the lethal dose of lipopolysaccharide (LPS) from Pantoea agglomerans given by the intradermal (i.d.) route is about 300 times greater than that given by the intravenous (i.v.) route, and that 400 micrograms/kg of LPS administered i.d. significantly suppressed metastasis whereas administered i.v., it did not. To learn the specific mechanism involved in this i.d. administration, the fate of LPS at the skin following administration and the concurrent production of endogenous tumor necrosis factor (TNF) in serum was examined. Histological observation following the i.d. administration of LPS (40 micrograms/kg) revealed neutrophiles in the skin 6 hours later. After 24 or 48 hours inflammatory cells were assembled at the site of injection. Endogenous TNF activity was found in the skin 24 hours after the injection and was significantly detectable even after 48 hours. Endogenous TNF was induced around tumor lesions of Meth A fibrosarcoma, MH134 hepatoma and Lewis lung carcinoma by treatment of LPS administered i.d. Taken together, these findings suggest that the antitumor activity of i.d. administered LPS results from the continuous supply of a small amount of this substance producing free TNF and activating inflammatory cells such as macrophages having membrane bound proTNF on their surface from the injected site to the tumor lesion for more than 48 hours.