Cardiopulmonary bypass surgery may be complicated by a systemic inflammatory reaction, which has been ascribed to the activation of complement. For such activation, the choice of priming solution for the heart-lung machine may be of importance. The peripheral blood of two groups of 10 patients, either exposed to dextran 70 or to plasma as priming solutions, was therefore studied pre-, per-, and postoperatively. The study confirmed that activation of complement is a consistent phenomenon during cardiopulmonary bypass surgery and that the activation involves both the early and the late phase of the complement cascade. The increase in the plasma concentration of the C3 activation products C3c and C3dg was significantly higher in the plasma group than in the dextran group, while there was no difference in the increase in the concentration of the terminal complement complex SC5b-9.