A 68-yr-old male with chronic lymphocytic leukemia (CLL) presented with splenomegaly and skin infiltration but no lymphadenopathy. The peripheral blood WBC cound was 300 x 10(9)/liter, with 95% small mature- appearing lymphocytes that were E-rosette positive and EAC-rosette negative. Further characterization of the patient's cells was performed using antisera with known lymphoid sub-population specificity. Anti- p23,30, which reacts with normal circulating B cells but not with T cells or thymocytes, was unreactive with the patient's cells. Anti-311, which reacts with both thymocytes and circulating T cells, was reactive with the patient's cells. Anti-Bk, which reacts only with thymocytes and not with circulating T-cells, failed to react with the patient's cells. The enzyme terminal deoxynucleotidyl transferase, present in thymocytes but absent for circulating T-cells, was also absent from the patient's lymphoid cells. Multimarker analysis therefore showed a mature T-lymphocyte phenotype on this patient's leukemia cells. Further functional analysis will probably show that such cells represent clonal expansion of a mature T-cell subpopulation, analogous to the B-cell clonality of common-variant CLL.