A proliferating cell nuclear antigen (PCNA) was identified with autoantibodies from a patient with systemic lupus erythematosus. Specific antibodies were purified by affinity chromatography in which Novikoff hepatoma nucleolar proteins were conjugated to Sepharose-4B. The purified anti-PCNA antibodies produced bright nucleolar fluorescence in tumor cells as shown by indirect immunofluorescence. PCNA was found in nucleoli of human cell lines, HeLa, Hep-2, and Namalwa, and a solid human renal and a prostate carcinoma. Both strong and weak nucleolar fluorescence areas were found in the renal and prostate carcinoma indicating that there are varying degrees of proliferation among tumor cells. Two human colon carcinoma cell lines, omega (an aggressive, fast-growing clone of human colon carcinoma cell line HCT 116) and CBS [a slow-growing human colon carcinoma cell line (group 3)], with different growth rates were compared. The fast-growing colon carcinoma cells, omega, exhibited a higher percentage of nucleolar fluorescence (28.5%) than that of the slow growing colon cells (13.6%). By enzyme-linked immunosorbent assays, the omega cell extract had a higher PCNA antigen content (2.8-fold) than that of the CBS cell extract which, in turn, was higher than that of human liver extract. PCNA was also found in a human fetal lung fibroblast cell line (IMR-90). Very weak or negative nucleolar fluorescence was observed in several normal human tissues including liver, kidney, prostate, and cheek cells. Nucleolar fluorescence was also observed in rat Novikoff hepatoma cells. Although normal rat livers do not have PCNA nucleolar fluorescence, nuclear and nucleolar fluorescence were observed at 18 hr after partial hepatectomy.