B- and T-cell markers were studied in a patient with prolymphocytic leukemia, a rare variant of chronic lymphocytic leukemia. Thymus-derived features were identified on the membrane of the neoplastic lymphocytes using the following cellsurface markers: Heterologous T-cell antigen, sheep erythrocyte receptor, surface immunoglobulin, complement receptor, Fc receptor and mouse erythrocyte receptor. Cytogenetic studies of leukemic cells from unstimulated and mitogen-stimulated cultures revealed a consistent karyotype characterized by marker chromosomes and a decreased chromosome number, whereas chromosomal analysis of hair root cells yielded a normal karyotype. A uniform expression of T-cell antigens measured on single leukemic cells by quantitative microphotometric immunoautoradiography correlated with the cytogenetic findings which are compatible with a descent from one progenitor cell.