A nuclear antigen associated with cell proliferation (proliferating cell nuclear antigen [PCNA]) and blast transformation is recognized by autoantibodies in the sera of some patients with systemic lupus erythematosus. Using this autoantibody as a reagent, PCNA was purified 120-fold by ammonium sulfate fractionation, DEAE chromatography, and Sephadex G200 gel filtration. The antigenicity of PCNA was sensitive to trypsin but resistant to ribonuclease and deoxyribonuclease, suggesting that the antigenic determinant resided in protein and not nucleic acids. PCNA was inactivated at 56 degrees C for 30 min. Isoelectrophoretic focusing showed that the pI was 4.8. Analysis of immunoprecipitates on polyacrylamide gels showed the presence of IgG heavy and light chains and a single polypeptide band of 33,000 mol wt. This polypeptide band was the reactive antigen in immunoblotting (Western transfer) assays.