Two surgeons, in whom liver damage developed after occupational exposure to sub-anaesthetic doses of halothane, were found to have a circulating antibody which reacted specifically with halothane-altered hepatocyte membrane components. This antibody has been found previously only in those patients in whom severe hepatic necrosis developed after exposure to halothane and in no other form of liver injury. It may provide a specific diagnostic marker in patients in whom there are other possible causes of liver damage and could, therefore, remove the need for a challenge exposure and its attendant risks.