Several herbal remedies have produced hepatitis in humans. The medicinal plant, germander, was recalled after its use as an adjuvant to slimming diets resulted in an epidemic of hepatitis in France. We studied the hepatotoxicity of germander in isolated rat hepatocytes. A crude fraction containing the diverse furano diterpenoids of germander, or the purified main constituents of this fraction, teucrin A and teuchamaedryn A, were hepatotoxic (correction for hepatototoxic), but not fractions containing more polar or lipophilic constituents. [3H]Teucrin A covalently bound to hepatocyte proteins. The furano diterpenoid fraction decreased cell glutathione and cytoskeleton- associated protein thiols, and led to formation of plasma membrane blebs and cell demise. Pretreatment of male rats with troleandomycin, an inhibitor of cytochrome P450 3A (CYP3A), slowed the depletion of glutathione and decreased toxicity, whereas dexamethasone, an inducer of CYP3A, had opposite effects. Female rat hepatocytes, which poorly express CYP3A, exhibited little toxicity, unless the animals were treated with dexamethasone. Feeding male rats with a sulfur amino acid- deficient diet decreased cell glutathione and enhanced toxicity, whereas supplementation of the standard diet with cystine had opposite effects. We conclude that the furano diterpenoids of germander are activated by CYP3A into electrophilic metabolites that deplete glutathione and cytoskeleton-associated protein thiols and form plasma membrane blebs. We suggest that studies in isolated hepatocytes be included in the preclinical assessment of herbal remedies.