The peripheral blood of three patients with otherwise typical hairy-cell leukemia (HCL), but with a substantial number of circulating cells expressing both light chain restricted surface immunoglobulin (SIg) and sheep erythrocyte (E) receptor, was studied with a panel of monoclonal antibodies. The presence of hybrid B-T surface features was confirmed and, although as in previous studies of E+SIg+ proliferations, the precise phenotypes varied at different times of study, E+ non-T SIg+ and E+T+SIg+ populations were at times present in all three patients together with typical B hairy-cell (HC) and phenotypically normal T-cell populations. In one patient an E-OKT11+SIg+ population was also observed. Extensive precautions were taken to exclude spurious explanations for these hybrid phenotypes. These results are discussed in relation to the recent demonstration that both HCs and normal B cells can express T-cell features after appropriate in vitro stimulation.