Four different functions of the immune defence were studied in six men undergoing open-heart surgery with cardiopulmonary bypass (CPB). Pre-operatively, a few hours after and three days after surgery the following tests were performed: (I) in vivo phagocytic and metabolic functions of the reticuloendothelial system (RES), (II) haemolytic activity of blood monocytes, (III) nitroblue tetrazolium (NBT) reduction by granulocytes, and (IV) bactericidal capacity of granulocytes. Compared to the pre-operative values, the RES functions were unchanged postoperatively, whereas there was a significant increase in the haemolytic activity of monocytes and in the NBT reduction of granulocytes. The capacity of the granulocytes to kill bacteria was normal a few hours after surgery, but significantly increased on day 3. No infectious complications occurred and all patients recovered uneventfully. These results suggest that, at the present time, open-heart surgery under CPB is accompanied by an increased activity of granulocytes and monocytes in the early postoperative period.