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WITH THE ASSISTANCE OF D. EIIKLI('H il lie Ikpczrfrrirnl of &ology, Tke Jo/k?u Bop1,zn.v Unzzievstly and T~P 1) eparlment of C~i n c c c i~Laboratorzes, Xercy Hospetal, Balterizorr, Maryland INTRODTJC'I'TOISThe recent isolation and chemical chaiuacterization of vitamin K (tlie antihemorrhagic vitamin) has stimulated rcnewcd interest in the study of factors involved in tlie phenomenon of blood coagulation. Currcrit investigations have shown that prothrombin is one of the most essential constituents to this process. To date, the chemical nature of prothrombin is unknown, its magnitude is ascertained in terms of the coagulation activity of blood or plasma. Simplified procedures for measuring thc prothrombin activity of blood liave I m n introduced with the resnll that many investigations are now extant. Warner, Rrinkhous and Smith ( '36) suggest that the blood clotting mechanism in lower animals is a rutlimentayv mechanism and that there is a gradual evolution through the various classes of vertebrates. Warmblooded animals, i.e., mammals and to a lcsscr extent birds, have beer1 the subject of rather intensive investigation. Blood eoagnlation in colilblooded animals and hiberiiating vertebrates on tlie other liand has received very little attention.Two distinct lines of evidence are available for experimental derangcment of the coagulation mechanism of blood. IIemorrhage resulting from a reduced prothrombin activity of the blood was first produced in birds (chicks) by t i dietary (vitamin I<) deficiency (Dam, '35; Almquist, '36). This same condition has becn obtained in dogs surgically (bilc fistula, Smith et al., '36) and complete hepatcctomy (Andrus et al., '40) and chemically (chloroform and phosphorus poisoning, Smith et al., '36) indicating that the liver plays a significant role in this proccss.