Antimitochondrial antibodies were detected by fluorescent antibody technique in seven of nine sera from patients with jaundice after halothane anesthesia, and from four of seven patients with jaundice after therapy chlorpromazine. They were not observed in jaundized patients treated with other drugs (methyldopa, aminosalicylic acid, chlorpropamide, perphenazine), in patients in whom jaundice did not develop after receiving halothane or chlorpromazine, or in patients with viral hepatitis. The presence of antibodies may help differentiate between viral hepatitis and some forms of jaundice after drug administration.