This is a monograph on the author's researches in blood clotting. This work has not been well known in the United States, and the book gives the opportunity to study and to appreciate the author's contributions to an important chapter of medicine. His experiments refute the belief that the blood platelets initiate the clotting by releasing thromboplastin. He shows that the plasma contains a prothromboplastin (he names it prothrombokinin) which becomes thromboplastin by the action of a substance discovered by the author, named thrombokatalysin. This occurs in low concentration in the circulating blood and may be identical with a fraction of trypsin. The blood of hemophiliac persons is deficient in thrombokatalysin, and the addition of this substance to hemophilic blood makes it clot in normal time. The last part of the monograph deals with experiments on the anticoagulants hirudin, heparin and apicur (bee venom).