The major focus of the studies proposed is the quantitative description of certain of the controls that regulate the rate and extent of production of Factor Xa and thrombin during the coagulation of plasma. These will be studied first in purified systems and in systems based on bovine plasma. The final goal, however, is the kinetic measurement of these controls during the clotting of human plasma. The first area of study is the controls that operate in the tissue factor-dependent activation of Factors IX and X. This is a complex area, but the studies will be largely addressed to two questions: (1) what are the relative kinetics of activation of Factors IX and X, with particular reference to initiation with low levels of tissue factor; (2) what is the effect on these kinetics of the feedback activation of Factor VII by Factor Xa? The second area concerns the kinetics of inhibition of Factor Xa in plasma by both antithrombin and non-antithrombin inhibitors. The study of Factor X activation above will define the kinetics of total enzyme generation, while this area will define the kinetics of disappearance. The third area is similar study of the kinetics of thrombin inhibition during clotting of plasma. New methods, largely immunochemical, are proposed for the measurement of the gross generation of thrombin during coagulation and these will allow the kinetic analysis of generation versus inhibition. Studies also will be made of the effects of plasma components other than inhibitors on the kinetics of thrombin inhibition, with specific reference to thrombin substrates and platelets. The main thesis of this study is that the "coagulant effect" of Factor Xa or thrombin during clotting must be a function of the integrated area under the respective generation/inhibition curves, and that this area is determined not so much by the amount of Factor X or prothrombin activated, but by the kinetics of generation and inhibition.