The postoperative changes in serum aspartate aminotransferase (SGOT), serum alanine aminotransferase (SGPT), and lactate dehydrogenase (LDH) levels after major lung operations were studied in patients some of whom received halothane and some of whom did not. There were no differences between the two groups of cases. It seemed possible that the rises in SGOT and LDH which occurred quite frequently were an indication that tissue autolysis was occurring in the body.Retrospective statistical studies have largely discounted the risk of the development of massive hepatic necrosis as a complication of halothane anaesthesia (National Halothane Study, 1966;Mushin, Rosen, Bowen, and Campbell, 1964;Vickers and Dinnick, 1965;Henderson and Gordon, 1964). This form of investigation, however, involves a collection of data from a large number of sources with the inevitable accompanying variability in the material. One of the conclusions of the National Halothane Study was that prospective investigations of the toxicity of halothane would now be justified, and the present paper attempts this assessment.The most obvious method of detecting minor liver damage is the determination of the transaminase content of the serum of patients in the postoperative period, for such enzymes do not appear in appreciable quantity unless liver cells have been destroyed and are undergoing autolysis. A certain amount of work in this field has already been carried through (Ayres and Williard, 1960;Lawrence and Schulkins, 1956), including enzyme studies on patients to whom chloroform had been given (Fote,