Continuous flow filter paper electrophoresis, as developed by Svensson and Brattsen (1), Grassmann and Hannig (2-4) and Durrum (5), offers a method of plasma fractionation in which protein denaturation appears minimal. This study involves the testing of such fractions for biological activity in various coagulation systems. With the exception of fibrinogen, the coagulation proteins are present in plasma in minute amounts which, at the present time, can be assayed only by their activities in specific clotting systems. Many of these coagulation factors are readily denatured; thus, rigid control with particular attention to temperature is necessary to obtain good recovery of biological activity.Previous electrophoretic studies concerning distribution of plasma coagulation proteins have employed paper electrophoresis with elutions from cut strips. Owen and McKenzie (6) found prothrombin to coincide with a2 globulin, proconvertin (stable conversion factor) with ft globulin, proaccelerin (labile conversion factor) in a variable area between y and f8 peaks, antithrombin between /3 and a2, and fibrinogen remaining at the point of application. Using the same technique, Frick and Hagen (7) found Hageman factor activity in a fraction located between ft and -y globulin. On the other hand, the electrophoretic mobility of purified prothrombin was described by Seegers, McClaughry, and Fahey (8) as approximately equivalent to that of a, globulin. The