Plasma concentrations of the granulocyte cell marker calprotectin were assessed during operation and 24 hours postoperatively in patients undergoing coronary artery bypass grafting with cardiopulmonary bypass, abdominal aneurysmectomy with implantation of an aortic graft, or thoracotomy without implantation of synthetic material. The concentration of calprotectin increased significantly (p < 0.01) in all three groups. Ten of the 30 patients in the group undergoing cardiopulmonary bypass received methylprednisolone at the start of the operation. No difference in calprotectin concentration was seen between the two subgroups (p > 0.05). Plasma concentration of calprotectin was shown to increase rapidly in patients undergoing cardiopulmonary bypass and aneurysmectomy, in whom complement activation also took place. However, the calprotectin concentration increased slowly during the operation and the postoperative period in patients undergoing a thoracotomy, in whom complement was not activated. At wound closure the calprotectin concentration was significantly elevated in the cardiopulmonary bypass and aneurysmectomy groups compared with the thoracotomy group (p < 0.05). The calprotectin concentration remained elevated during the postoperative period in all three groups. Our results indicate that calprotectin may serve as a suitable cellular marker when the biocompatibility of artificial surfaces is studied.