A study of lymphocyte cytotoxicity for rabbit hepatocyte cultures in 15 patients with untreated chronic active hepatitis showed positive results in all cases, both HBsAg positive and negative. After immunosuppressive therapy cytotoxicity became negative and remained negative, in four of nine patients followed serially. In 51 patients established on therapy for periods from three months to 12 years, cytotoxicity was negative in 19 and all patients are currently alive. However, in the remaining 32 patients in whom cytotoxicity was positive there has been a 34% mortality. Cytotoxicity remained persistently positive in 12 of 15 patients followed serially, and persistently negative in seven of nine. Cytotoxicity showed a significant association with histological disease activity, especially the extent of piecemeal necrosis, but not with biochemical tests of liver function, immunoglobulins, or autoantibodies. The basis of this cytotoxicity test is an antibody dependent cell-mediated autoimmune reaction directed against a liver specific protein, and the results suggest that in some cases immunosuppressive therapy is followed by control of this reaction. It may be possible to stop therapy in these patients, but in those in whom the reaction continues, as shown by continuing cytotoxicity, the prognosis is not as good and the use of other drug schedules would seem worthy of trial.