In using the rat hepatocytes isolated and treated with formalin as antigen, IgG class antibodies to the liver cell membrane (LM-Abs) in sera of patients with chronic active hepatitis were examined by both indirect immunofluorescence (IF) and solid-phase enzyme immunoassay (EIA). Specificity of the reaction in IF and EIA was confirmed by an absorption test using different tissues and by inhibition test using the liver specific protein. Indirect IF test using isolated hepatocytes showed the linear staining in eight of 14 sera of patients with autoimmune chronic active hepatitis (CAH). Mean binding values and positive frequency of LM-Abs to the isolated hepatocytes which were detected by EIA were 4.18 +/- 2.99 and 71.4% (10/14) in autoimmune CAH, 3.16 +/- 2.50 and 50% (6/12) in primary biliary cirrhosis, 1.40 +/- 2.12 and 28.6% (4/14) in non-A, non-B, chronic hepatitis (NANB-CH) with serum gamma-globulin showing more than 2.0 gr/dl, 1.02 +/- 1.30 and 0% (0/13) in NANB-CH with gamma-globulin showing less than 2.0 gr/dl, 1.04 +/- 1.20 and 0% (0/15) in HB-CH, and 1.80 +/- 3.60 and 25.0% (3/12) in SLE. Titers of LM-Abs in serum of a autoimmune CAH patient decreased significantly after treatment with prednisolone, but their titers did not correlate with the activities of anti-nuclear antibodies or anti-smooth muscle antibodies. From these results, it was considered that some of NANB-CH patients with both LM-Abs and higher serum gamma-globulin more than 2.0 gr/dl might have some autoimmune characters similar to CAH.