Cell surface markers and the responses of lymphocytes to T- and B-cell mitogens were studied in 10 patients with CCL. T cells were identified as cells rosetting with sheep red blood cells (SRBC), and S-Ig was used as a marker for B lymphocytes. Most cells from all patients had a detectable amounts of S-Ig, and the percentage of cells rosetting with SRBC was low in all cases. Of the lymphocytes from these patients, 3-74% (mean 33%) were positive for the acid esterase (ANAE), which has been claimed to be a T-cell marker. However, some patients had cells that were positive for both S-Ig and ANAE. Acid esterase staining is therefore not a valid T-cell marker in chronic lymphocytic leukaemia. In cultures containing the T-cell mitogen leucoagglutinin (LA) and the T- and B-cell mitogen pokeweed mitogen (PWM) the reactivity of the lymphocytes was low. The cells responded vigorously to the T- and B-cell mitogen protein A (PA); however, the response was serum-dependent, being strong in a culture medium containing foetal calf serum (FCS), but impaired in the presence of human AB serum. Only 1 patient had cells that responded to the B-cell mitogen LPS.