Despite adequate anticoagulation, thrombi may develop onto any foreign surface subjected to circulating blood. Although the efficiency of membrane oxygenators can decrease, distal embolization can be more critical and dangerous. Scanning electron microscope observations carried out on the Travenol Modulung oxygenator used both on dogs and on patients for prolonged partial cardiopulmonary by-pass pointed out the following observations. Caution must be observed when translating the results from dogs to human beings. Heparin management can be delicate and critical. The problem of blood transfusion can be extremely important and must be extensively investigated.