Antibodies to cytoplasmic microfilaments, intermediate filaments (vimentin filaments), and microtubules which comprise the cytoskeleton of the cell were assayed in sera from 23 patients with HBsAg-negative chronic active hepatitis and 1 with HBsAg-positive chronic active hepatitis, 15 patients with primary biliary cirrhosis, 20 patients with alcoholic liver disease, and 32 healthy controls. The cytoskeleton antibodies were assayed by indirect immunofluorescence technique using vinblastine-treated cultured human embryonic fibroblasts as substrates. There was a significantly increased incidence of cytoskeleton antibodies in patients with liver disease as compared to the control group. Antibodies to microfilaments were found frequently in sera from patients with chronic active hepatitis (67%) and primary biliary cirrhosis (53%) but were rare in sera from patients with alcoholic liver disease (25%) and in control sera (3%). On the other hand, antibodies to microtubules were found in 50% of sera from patients with alcoholic liver disease but in only 7 to 13% of sera from other groups. Intermediate filament antibodies of IgG or IgA class were found only in patient sera whereas intermediate filament antibodies of IgM class were found in the majority of sera in all groups including control sera. The highest titers of intermediate filament antibodies were seen in primary biliary cirrhosis and chronic active hepatitis. The production of cytoskeleton antibodies may be due to the reorganization or destruction of cytoskeletal structures in the liver.