This study evaluated the effects of leucocyte depletion on reperfusion injury during cardiopulmonary bypass in 26 patients who underwent coronary artery bypass graft. In 10 patients leucocytes were continuously separated from the circulating blood; in 16 leucocytes were undepleted. Leucocyte count and serum creatine phosphokinase activity (and that of its isoenzymes) were measured before and after aortic cross-clamping; in addition the cardiac index and required catecholamine dosage during the early postoperative period in the two patient groups were compared. Mean(s.d.) leucocyte counts immediately after surgery were 8622(2887) microliters-1 in patients who were leucocyte depleted and 12,175(5303) microliters-1 in controls (P < 0.05). Mean(s.d.) creatine phosphokinase activity in patients with leucocyte depletion was significantly lower than in controls (334(121) versus 821 (356) units l-1; P < 0.05) immediately after surgery. Moreover, mean(s.d.) catecholamine dosage was significantly less in the leucocyte-depleted group than in controls (1.1(1.9) versus 4.9(2.2) micrograms kg-1 min-1; P < 0.05), whereas the cardiac index was significantly higher (3.3(0.5) versus 2.3(0.4) l min-1 m-2; P < 0.05). These results suggest that, in the clinical setting, leucocyte depletion during cardiopulmonary bypass provides effective protection against reperfusion injury.