Sera obtained from patients with autoimmune liver disease were screened in indirect immunofluorescence microscopy for the presence of autoantibodies. Patients' sera, which strongly stained nuclei (ANA) with peripheral accentuation, were used for further experiments to define the corresponding antigen(s). Nuclei and nuclear subfractions were isolated from HeLaS3 cells and used as antigen source. Immunoblotting experiments were performed after separation of nuclear proteins by one- and two-dimensional polyacrylamide gel electrophoresis. Some ANA positive sera recognized the nuclear protein with molecular mass of approximately 200 kDa. Further analysis revealed that the patients' sera reacted with gp210, an integral protein of the nuclear pores. The incidence and clinical significance of these antibodies is discussed.