Investigation of humoral immunity against hepatocellular membrane antigens in patients with chronic active hepatitis and other liver diseases showed two different immunofluorescence patterns of IgG on hepatocyte membranes. A linear pattern was seen in HBsAg-negative hepatitis, but HBsAg-positive cases and some of protracted, acute hepatitis B had a granular pattern. In patients with IgG bound to hepatocytes, continuing necrosis of parenchymal liver cells was seen. Conversely, hepatocytes without bound IgG were found in cases of chronic active hepatitis in remission, acute viral hepatitis without HBsAg and chronic persistent hepatitis, in "healthy" HBsAg-carriers and in patients with fatty liver or alcoholic cirrhosis. A liver-membrane autoantibody in serum, proved by fixation on membranes of isolated rabbit hepatocytes, could be demonstrated only in HBsAg-negative chronic active hepatitis with elevated IgG-concentrations. The results support the existence of different pathogenetic types of chronic active hepatitis, a so-called autoimmune type and a hepatitis virus-B-induced type.