A patient with T-cell chronic lymphocytic leukemia (T-CLL) is reported whose cells demonstrate in vitro suppression of normal lymphocyte mitogen stimulation. The patient, who remains in Rai's clinical stage 0 on no therapy after more than 24 mo of observation, has shown a less aggressive clinical course than is usually attributed to T-CLL. His peripheral blood lymphocytes (PBL) were characterized by functional assays as well as surface markers. Over 90% of the patient's PBL formed rosettes with sheep erythrocytes and were lysed by two T-cell-specific antisera plus complement, while less than 1% bore surface immunoglobulins, and only 3% had complement receptors. In addition, 45% of the PBL demonstrated Ia-like antigens, more than 50% expressed a receptor for the Fc portion of IgG(T gamma), and most of the sheep erythrocyte rosettes were inhibited by theophylline. The patient's cells failed to respond to several mitogens and they caused marked suppression of lymphoproliferative responses to normal PBL to phytohemagglutinin (PHA) and concanavalin A (Con-A). The patient's lymphocytes also exhibited antibody-dependent cytotoxic activity (ADCC) against antibody-coated nucleated target cells, but lacked demonstrable natural killer (NK) activity. This patient's T-CLL cells appear to represent the clonal expansion of a subset of T cells with a previously undescribed pattern of suppressor and cytotoxic activities.