Isolated rat hepatocytes were used to determine the cytotoxicity of peripheral blood mononuclear cells (PBM) from patients with alcoholic liver disease (ALD) and chronic active hepatitis (CAH). The specificity of the cytotoxic effect on liver cells was monitored using rat kidney cells. PBM from patients with CAH and ALD showed increased spontaneous cell-mediated cytolysis (SCMC) for hepatocytes. The SCMC against kidney cells was comparable for the patient groups and controls. Incubation of liver cells with antibody to human liver-specific protein significantly increased the cytotoxic activity of control PBM but did not block cytotoxicity by PBM of patients with CAH. Incubation of PBM from untreated CAH patients with thymosin fraction 5, a polypeptide extract of the thymus gland, significantly lowered cytotoxicity. Our findings suggest that rat liver cells provide a model for studies of cell-mediated immunity in human liver disease and that thymosin fraction 5 decreases the cytotoxic activity of sensitized lymphocytes in CAH.