We studied the in vivo antitumor effects of natural human tumor necrosis factor-alpha (nHuTNF-alpha) and natural human interferon-alpha (nHuIFN-alpha), both of which were produced by HVJ (hemagglutinating virus of Japan)-stimulated acute lymphatic B cell leukemia line, BALL-1 cells. To clarify the interaction between nHuTNF-alpha and nHuIFN-alpha, we used novel experimental models of lung metastasis and intraabdominal carcinomatosis which we developed in nude mice using a human tumor line, RPMI 4788. While the intravenous administration of nHuTNF-alpha or nHuIFN-alpha alone inhibited lung metastasis, the two cytokines given in combination synergistically inhibited lung metastasis. In a comparative study, nHuTNF-alpha and recombinant human interferon-gamma (rHuIFN-gamma) in combination also synergistically inhibited lung metastasis. Treatment with nHuTNF-alpha and nHuIFN-alpha combined significantly prolonged the survival of nude mice with intraabdominal carcinomatosis. Complete regression of five different human tumor xenografts was achieved by the simultaneous intratumoral injection of nHuTNF-alpha and nHuIFN-alpha. Histological examination revealed that tumor cell lysis occurred 24 h after the intratumoral administration of the cytokines. No significant signs of toxicity to nude mice were observed at any dose tested. The synergism of nHuTNF-alpha and nHuIFN-alpha may allow treatment at a relatively low dose range, thus minimizing side effects. The wide range of anticancer activity of these agents may provide better therapeutic efficacy. The in vivo assay systems which we have developed are useful for the analysis of the biological activities and interactions of cytokines and chemotherapeutic drugs.