Fifteen patients with chronic lymphocytic leukaemia (CLL) were studied. The diagnosis was made by a combination of clinical findings, peripheral blood morphology and cell surface markers. The Rai clinical staging (Rai et al, 1975) was used to classify all patients. Serum immunoglobins, B and T cell lymphocytes, TG and TM cell populations were evaluated in all patients before commencing treatment. No correlation was found between clinical staging and the presence or absence of hypogammaglobulinaemia. The ratios of TG to TM cells were abnormal in all patients and this abnormality paralleled the stage of the disease. While there was significant difference in the proliferative responses of highly purified T cells from patients and controls at 1 micrograms and 2 micrograms/ml of Concanavalin A (Con A) (P values = P less than 0.005 and P less than 0.01 respectively). There was no significant difference at dose 5 micrograms/ml. The ability of T cells to suppress allogeneic PBMC responses to Con A was dependent on T cell purification procedures. The significance of the results and the possible role of T cells in the pathogenesis of CLL is discussed.