It is one of the main charms of blood coagulation that advanced biochemistry is continuous to everyday medical practice and vice versa. Common clinical facts hide the most enchanting enzymology. The prothrombin time (PT, thromboplastin time, Quick time) and the activated partial thromboplastin time (APTT) are perfect examples. Both are commonly used in the clinical laboratory as global tests for the function of the coagulation system and for the control of anticoagulant therapy. They are at the basis of the classical one-stage clotting factor determinations. Practice has taught us that the PT is perfectly suited for the control of anticoagulant therapy but not for measuring the effect of heparin, whereas the APTT can serve both purposes. Only recently was it found out why: the length of the APTT is primarily determinated by the time necessary for the activation of factor VIII by thrombin [1]. Now thrombin, of course, is the final enzyme of the coagulation cascade, whereas factor VIII is one of the factors necessary for its formation. The product actiyates one of the proteins instrumental in its own formation: a clearcut case of feedback activation. Obviously, if thrombin is quickly O l99l S. KargerAG, Basel 030 | -0 | 47 I 9 | | 02 | 4-Ot 89s2.7 5 / O