Previous investigations have shown that antibodies in sera from patients with halothane hepatitis recognize neoantigens, expressed in livers of halothane-exposed rabbits and rats, which consist of a halothane metabolite bound covalently to specific microsomal proteins. These studies have suggested that the patients' antibodies may play a role in the pathogenesis of the hepatitis. In the present investigation, human liver biopsy samples were analyzed using an immunoblotting method to seek evidence for expression of halothane-induced neoantigens in humans. Sera from four patients with halothane hepatitis, which recognized halothane-induced rabbit liver neoantigens of 100, 76 and 57 kD, reacted strongly with antigens of very similar molecular weights that were expressed in livers from two patients who had died of cardiac failure following recent anesthesia with halothane. The antigens were not expressed in normal human liver or in livers from three patients who died of cardiac failure following anesthesia with agents other than halothane. The human antigens were not recognized by antibodies present in various control sera. Recognition of the 100- and 76-kD human antigens by the patients' antibodies was greatly reduced by absorption of sera with liver microsomes from halothane-exposed rabbits, but not by absorption of sera with control rabbit microsomes. These results indicate that humans exposed to halothane express liver neoantigens which are analogous to the halothane metabolite-protein neoantigens characterized previously in halothane-exposed animals.