Intraoperative application of the proteinase inhibitor aprotinin allows to drastically reduce blood loss during and after cardiopulmonary bypass operation. The side effects of this therapy (if any) have not been systematically registered and a possible interaction with heparin is not excluded. Such an interaction seems to have occurred in one of our patients. He developed a resistance to heparin shortly after prophylactic administration of aprotinin and maintained it for about one hour after discontinuation of the aprotinin. Prophylactic and therapeutic implications of this observation are briefly exposed.