Tumor necrosis factor α (TNF-α) binding to the TNF receptor (TNFR) initiates apoptosis and simultaneously activates the transcription factor, nuclear factor-κB (NF-κB), which suppresses apoptosis by an unknown mechanism. Pretreatment with TNF-α or interleukin-1β (IL-1β), which activated NF-κB in the liver, dramatically prevented TNF-α-induced liver-cell apoptosis in D-galactosamine (GalN)-sensitized mice, but not anti-Fas antibody-induced hepatotoxicity. This protective effect of TNF-α continued for 5 hours after TNF-α administration, a time course similar to that found in NF-κB activation after TNF-α administration. In mice treated with adenoviruses expressing a mutant form of IκB, the antiapoptotic effect of TNF-α was inhibited in part. Prior TNF-α administration was not found to block the activation of caspase-8, although caspase-3 was inhibited in mice treated with TNF-α plus GalN/TNF-α compared with mice treated with GalN/TNF-α. These results indicate that TNFR and Fas independently regulate murine apoptotic liver failure, and that a rapid defense mechanism induced by the activation of NF-κB blocks death-signaling at the initiation stage of hepatic apoptosis mediated by TNFR, probably downstream of caspase-8, but not by Fas.