A 68 yr old [human] male with chronic lymphocytic leukemia (CLL) presented with splenomegaly and skin infiltration but no lymphadenopathy. The peripheral blood WBC [white blood cell] count was 300 .times. 109/l, with 95% small mature-appearing lymphocytes that were E[erythrocyte]-rosette positive and EAC[E, antibody, complement]-rosette negative. Further characterization of the patient''s cells was performed using antisera with known lymphoid subpopulation specificity. Anti-p23,30, which reacts with normal circulating B [bone marrow-derived] cells but not with T [thymus-derived] cells or thymocytes, was unreactive with the patients''s cells. Anti-311, which reacts with 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 from circulating T cells, was also absent from the patient''s lymphoid cells. Multimarker analysis showed a mature T lymphocyte phenotype on this patient''s leukemic cells.