This little monograph contains some original contributions of the author to the confusing problems of blood coagulation. First a relatively simple two step method for the determination of prothrombin is described. Although the practical clinical significance of Quick's one step method is acknowledged, the theoretical foundation of the Quick test is severely criticized. After description of the influence of many factors on the determination of the prothrombin time, the effect of amino acids, ascorbic acid, taurin and cystein are particularly mentioned. Dihydroxyphenylalanine was found to inhibit coagulation in vitro and in vivo and is recommended for a clinical trial in thromboembolic disease. The book contains contributions of interest to the investigator in the field of coagulation.