Acetaminophen is a widely used analgesic and antipyretic drug that exhibits toxicity at high doses to the liver and kidneys. This toxicity has been attributed to cytochrome P-450-generated metabolites which covalently modify target proteins. Recently, acetaminophen, in its unmetabolized form, has been shown to affect a variety of cells and tissues, for instance, testicular and lymphoid tissues and lymphocyte cell lines. The effects on cell viability of acetaminophen at a concentration comparable to that achieved in plasma during acetaminophen toxicity have now been examined with a hepatoma cell line SK-Hep1, primary human peripheral blood lymphocytes and human Jurkat T cells. Acetaminophen reduced cell viability in a time-dependent manner. Staining of cells with annexin-V also revealed that acetaminophen induced, after 8 hr of treatment, a loss of the asymmetry of membrane phospholipids, which is an early event associated with apoptosis. Acetaminophen triggered the release of cytochrome c from mitochondria into the cytosol, activation of caspase-3, 8, and 9, cleavage of poly(ADP-ribose) polymerase, and degradation of lamin B1 and DNA. Whereas cleavage of DNA into internucleosomal fragments was apparent in acetaminophen treated SK-Hep1 and primary lymphocytes, DNA was only degraded to 50-kb fragments in treated Jurkat cells. Overexpression of the antiapoptotic protein Bcl-XL prevented these various apoptotic events induced by acetaminophen in Jurkat cells. Caspase-8 activation was a postmictochondrial event and occurred in a Fas-independent manner. These results demonstrate that acetaminophen induces caspases-dependent apoptosis with mitochondria as a primary target. These results also reiterate the potential role of apoptosis in acetaminophen hepatic and extrahepatic toxicity.