Introduction Heparin synthesis, metabolism, and fate has been an enigma for many years. The following paper is based on the results of a three-year biochemical and clinical study of this fascinating substance. New methods of study were established, including the synthesis and experimental use of radioactive heparin in an effort to elucidate its many problems. Heparin has been employed clinically for a number of years in the treatment of thrombosis. The rationale for its application is the fact that it is an extremely potent inhibitor of blood coagulation, and consequently it may be expected to be beneficial in thromboembolic diseases. Its effect as an anticoagulant is almost immediate, and it is therefore extremely useful where rapid action is required.1Heparin acts primarily on one of the later stages of blood coagulation, i. e., it inhibits the formation of thrombin from prothrombin and, to some extent, prevents thrombin from converting