A test of lymphocyte cytotoxicity for isolated adult rabbit hepatocytes has been performed using lymphocytes from 40 children with acute or chronic liver disease. Positive cytotoxicity was not observed in 26 children without liver disease and rarely in 13 children with disease affecting primarily the biliary tract. Temporarily positive tests were found in those with acute hepatocellular disease, but tests remained positive in patients with chronic active hepatitis, while liver function tests remained abnormal. Persistently positive test occurred in those with liver disease associated with alpha-antitrypsin deficiency. Such altered immunoresponsiveness could be an important pathogenic mechanism leading to chronic liver disease in childhood.