Under the effect of acetylsalicyl lysin, a postoperative hypercoagulability was ascertained with regard to shortened antithrombin and partial prothrombin times. The inhibition of aggregation was easily demonstrable with a high concentration of collagen in the test batch and showed a striking relationship to the occurrence of thrombosis. Under heparin prophylaxis, normal coagulability was maintained. Since disorders of hemostasis due to therapy occur sporadically, early postoperative raising of the dose of heparin should only be done under regular laboratory controll, to which activated PTT and antithrombin time is well suited.