Enough amounts of tumor necrosis factor (TNF) in mice serum for the therapy were observed by treatment with 100 units of recombinant human TNF-α (rHuTNF-α) followed by administration of OK-432 (a streptococcal preparation). Optimal time interval between rTNF and OK-432 to produce endogenous TNF was 3 h, and priming activity of rTNF persisted for at least 10 h. The same effect was observed using novel human recombinant TNF-SAM2(rHuTNF-SAM2) developed by our group. Production of endogenous TNF using rTNF-α or rTNF-SAM2as a priming reagent was almost equal among various mice strains. Induced TNF in mice serum was completely neutralized by anti-MuTNF antiserum, but not by anti-HuTNF monoclonal antibody. rMuTNF could also induce the priming state; however, the doseresponse kinetics of the priming effect to produce endogenous TNF was different between rHuTNFs and rMuTNF-α, suggesting species specificity among rTNFs used. The therapeutic effect against Meth A and MH134 tumors in mice treated by rHuTNFs in combination with OK-432 was superior to that by single administration of either OK-432 or rHuTNFs or by successive administrations of OK-432. Especially, the antitumor effect against MH134 hepatoma was superior to that of any other treatment using known biological response modifiers so far experienced. These results suggest that such combination antitumor therapy as rTNF together with OK-432 should be applicable to cancer patients.