In the present study, tumor necrosis factor-α (TNF-α) cytotoxicity is shown to be potentiated by ethanol exposure in vitroin the human hepatoma cell line, HepG2, and in rat primary hepatocytes. Exposure of HepG2 cells and primary hepatocytes for 48 hours to concentrations of ethanol ranging between 50 and 100 mmol/L significantly increased TNF-α cytotoxicity compared with cells treated with TNF-α alone. The cell killing was associated with, and dependent on, the development of the mitochondrial permeability transition (MPT). Two inhibitors of MPT pore opening, cyclosporin A and bongkrekic acid, prevented TNF-α cytotoxicity in the presence of ethanol. In addition to inhibiting cell death caused by TNF-α, blockade of MPT pore opening prevented mitochondrial depolarization, cytochrome c redistribution from the mitochondria to the cytosol, caspase 3 activation, and oligonucleosomal DNA fragmentation. Unlike the potentiation of TNF-α cytotoxicity by the translational inhibitor cycloheximide, ethanol promoted TNF-α–induced cell killing by a mechanism that was independent of caspase-8 activity. HepG2 cells overexpressing cytochrome-P4502E1 were even more sensitized by ethanol to induction of the MPT by TNF-α and the resultant cytotoxicity than wild-type HepG2 cells. In addition, primary hepatocytes isolated from chronically ethanol-fed rats showed enhanced susceptibility to TNF-α cytotoxicity compared with their isocalorically matched controls. Again as with the HepG2 cells, inhibiting MPT pore opening prevented the cytotoxicity of TNF-α in the primary hepatocytes isolated from ethanol-fed animals.