The present study employs a new technique for the study of human T-cell effector function in patients with chronic lymphocytic leukaemia: mitogen-stimulated interferon. Cultures of macrophages, T cell-enriched lymphocytes, or macrophages and lymphocytes combined were prepared from the blood of fourteen normal donors and five patients with chronic lymphocytic leukaemia. The effects of the mitogens, phytohaemagglutinin and pokeweed, on interferon production and lymphocyte transformation were studied and the following observations made: (a) T-cell effector and proliferative functions were depressed as evidenced by the absence of interferon and proliferative response to PHA and PWM at 3 days in vitro; (b) Three out of five patients showed no interferon or proliferative response at 6 days, thus indicating a B lymphocyte abnormality as well; (c) macrophages from both normal and leukaemic subjects augmented mitogen-stimulated lymphocyte interferon production and lymphocyte transformation. However, the addition of normal allogeneic macrophages to cultures of lymphocytes prepared from the patients did not restore the proliferative and interferon responses to normal levels.