Treatment with tumor necrosis factor (TNF) induces murine hepatocyte apoptosis in vitro and in vivo when sensitizing concentrations of toxins are present. The aim of this study was to investigate whether endogenously formed TNF contributes to liver failure caused by hepatotoxins. The extent of liver damage, induced by alpha-amanitin or actinomycin D (ActD), was examined under various experimental conditions, preventing the action of TNF on hepatocytes. TNF induced apoptosis of murine hepatocytes or human hepatoma cells in the presence of alpha-amanitin or ActD. TNF and alpha-amanitin induced such hepatotoxicity also in vivo in a synergistic way. After in vivo administration of high doses of ActD or alpha-amanitin alone, hepatic TNF-messenger RNA was increased and hepatocytes underwent apoptosis. A neutralizing antiserum against TNF-alpha prevented the liver injury. Hepatotoxicity of ActD or alpha-amanitin also was prevented by pretreatment of mice with low doses of the tolerizing cytokine interleukin 1. Mice deficient for the 55-kilodalton TNF receptor were protected from ActD-or alpha-amanitin-induced toxicity. Endotoxin-unresponsive C3H/HeJ mice also had liver failure after ActD treatment, and this damage was prevented by treatment with anti-TNF antiserum. Hepatotoxins such as alpha-amanitin may induce liver failure by an indirect mechanism involving sensitization of parenchymal cells toward endogenously produced TNF.