This investigation was undertaken in order to assess both the preventive and antiproliferative effects of tumor necrosis factor (TNF) in a hepatic metastasis model, by means of inoculation of mouse colon-26 tumor cells into the portal vein via the superior mesenteric vein in male CDF1 mice, aged 5 weeks. Continuous 10-day administration of natural human TNF-alpha (nHuTNF-alpha) following the tumor cell inoculation caused no reduction but rather an increase in the number of hepatic metastases. However, pretreatment with this preparation daily for 10 days before the inoculation caused a remarkable decrease in the number of hepatic metastases. This prophylactic effect was reversed by the intravenous administration of anti-asialo GM1 antibody 24 h before the inoculation. The result of immunoperoxidase staining of liver specimens suggested that organ-associated natural killer cells might play a role in the metastatic inhibition. An apparent antiproliferative effect on metastatic liver tumors was also recognized following injection of nHuTNF-alpha from the 10th day after the inoculation. Thus, TNF appears to have important effects upon the host immune system, acting against liver metastases.