[Anti-PCNA antibody].Antibodies to the proliferating cell nuclear antigen (PCNA) detected in patients with systemic lupus erythematosus (SLE) were allowed to react with a nuclear antigen expressed predominantly in proliferating cells such as cultured cells and mitogen-transformed cells. The characterization of both structure and function of PCNA has been studied. PCNA has been identified as a protein with a molecular weight of about 33 kD and isoelectric point of 4.8. The expression of PCNA increased in the cell from the late G1 and S phases of the cell cycle immediately preceding DNA synthesis. Recent studies have revealed that the auxiliary protein of DNA polymerase-d is identical to PCNA. Anti-PCNA antibodies can be detected by the methods of immunofluorescence (IF), double immunodiffusion (DID), counterimmunoelectrophoresis (CIE), enzyme-linked immunosorbent assay (ELISA) and immunoblotting (IB). Anti-PCNA antibodies were specifically detected in 2-5% of the patients with SLE by DID. In spite of their low frequency, anti-PCNA antibodies are useful as a clinical marker for SLE. Several reports indicated that patients with PCNA positive SLE showed a high frequency of renal and central nerve system (CNS) involvements and thrombocytopenia. In these patients, the anti-PCNA antibody titer was elevated before the development by proteinuria and the titer of anti-PCNA antibodies was decreased by treatment with corticosteroids. In addition, anti-PCNA antibodies have been known as a useful tool to detect activated lymphocytes and malignant proliferating cell.