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; these autoantibodies are precipitating antibodies and also react in immunofluorescence, a technique that was used to determine if PCNA might be expressed in leukocytes of patients with chronic myeloid leukemia (CML) during certain phases of their disease. At all times, a strong relationship was seen between the percentage of cells stained by anti-PCNA antibody and the percentage of blast cells in peripheral blood leukocytes (r) = 0.865, P less than .001. However, during blast crisis, certain cells that morphologically looked like myelocytes, metamyelocytes, and some cells with segmented nuclei stained with anti-PCNA serum. This staining, which remained nuclear in location, was less intense than in blast cells, suggesting low density of the antigen in these nonblast cells. This phenomenon was not observed in myelocytes or metamyelocytes obtained from patients in remission. These initial studies demonstrated that anti-PCNA can be used as a reagent to detect blast cells in CML crisis and also has the capability to detect PCNA in other cells associated with blast crisis.