Lymphocyte subpopulations from patients with chronic lymphocytic leukemia (CLL) and from normal age-matched controls were evaluated for their ability to participate in allogeneic and autologous mixed lymphocyte reactions (MLR). Unfractionated and T enriched lymphocyte populations from normal age-matched controls responded well to allogeneic stimulation. T enriched lymphocytes from patients will CLL also responded to allogeneic lymphocytes. B enriched lymphocytes from normal age-matched individuals produced a stronger stimulus in the allogeneic MLR than did unfractionated mononuclear cell populations. Unfractionated and B enriched lymphocyte subpopulations from patients with CLL were poor stimulators in the allogeneic MLR. In normal age-matched controls T enriched lymphocyte subpopulations were able to respond to autologous B enriched lymphocytes. Autologous mixed lymphocyte cultures from patients with CLL failed to demonstrate any activity.