We have studied the cytotoxicity against rabbit liver cells of lymphocytes from the peripheral blood of 71 patients with various liver diseases. The group with chronic active hepatitis and three patients with acute alcoholic hepatitis showed significantly higher mean values of lymphocytotoxicity (P less than 0.001) compared with the other patients with chronic persistent hepatitis, post-necrotic fibrosis and cirrhosis. Wilson's disease, and prolonged viral hepatitis. The mean cytotoxicity of these last groups did not differ significantly from controls. In four out of six patients with chronic active hepatitis a significant decrease of lymphocytotoxicity was found after immunosuppressive therapy with oral prednisolone. A good correlation between the lymphocytotoxicity test and histological signs of activity suggests that a cell-mediated immune aggression is present in this disease.