More than twelve years have elapsed since purified heparin preparations became available for clinical use.1The introduction of the second anticoagulant, dicumarol,® followed in 1941,2and since then a wide application of these two drugs resulted in all fields of medicine. Surgery, leaning heavily on the work of physiologists and biochemists, was first to utilize anticoagulant prophylaxis and therapy, since it is daily confronted with the phenomenon of clotting, and since it daily produces a postoperative state, which is characterized among other things by a transitory increase in the clotting activity of the blood.3 In this presentation I shall limit myself to personal observations on the vascular surgical service at St. Luke's Hospital, Chicago, for the past ten years. The reader may be referred to excellent collective reviews and monographs in the voluminous literature on the subject.4 TESTS OF CLOTTING ACTIVITY Ninety-seven normal persons were subjected