Sera of patients with chronic hepatitis delta virus infection stained the nuclear periphery in indirect immunofluorescence. Using proteins of isolated nuclei, isolated nuclear matrices, the nuclear pore complexlamina fraction and purified lamins A and C as antigen source in immunoblotting experiments, nuclear lamin C was identified as the reactive antigen. Most sera tested (8 of 10) recognized nuclear lamin C exclusively, but not the nuclear lamins A and B. Antibodies reacting with both nuclear lamins A and C, which share extensive sequence homologies, have been reported to occur in autoimmune hepatitis and primary biliary cirrhosis. The present findings suggest that the novel autoantibody associated with chronic hepatitis delta virus infection recognizes an epitope localized in the short carboxyterminal region of nuclear lamin C. (HEPATOLOGY 1990;12:1129–1133).