The profile of thymidine kinase isoenzymes was determined in peripheral blood lymphocytes from 14 patients with chronic lymphocytic leukaemia (CLL) and 31 controls. Twelve patients with indolent disease showed TK2 isoenzyme activity, while two patients in whom the disease evolved and two patients who presented with aggressive disease exhibited TK1 isoenzyme activity. The demonstration of TK1 activity in the peripheral blood lymphocytes of clinically aggressive CLL suggests that this isoenzyme may be a useful biochemical marker of such behaviour.