Although the expression of myeloid-associated antigen CD13 has been reported in aggressive B-cell chronic lymphocytic leukemia, its expression in other mature B-cell neoplasms appears to be rare. We report a 74-year-old female with B-cell prolymphocytic leukemia (B-PLL) expressing CD13 antigen. On admission, splenomegaly was noted. Hematological examination revealed a platelet count of 90 x 10(9)/l and a white cell count of 68 x 10(9)/l with 73% PLL cells. The hemoglobin concentration was 10.6 g/dl. A bone marrow aspirate showed a normocellular marrow with 64% PLL cells. Surface marker analysis of the PLL cells was positive for CD11b, CD13, CD19, CD20, CD24, HLA-DR, FMC7, mu and lambda. Simultaneous expression of CD13 and CD19 antigen was confirmed by dual color flow cytometry. Southern blot analysis of DNA from circulating mononuclear cells gave a rearranged band for the immunoglobulin gene (JH) but not for TCR-beta. Cytogenetic analysis of marrow cells showed an abnormal karyotype involving numbers 1, 7, 10, 12, 14, 15 chromosomes.