A spectrophotometric study of plasma coagulation is presented with a method for the determination of plasma coagulant activity, via the extrinsic pathway. Dilute citrated plasma was coagulated by recalcification after the addition of a very small amount of thromboplastin. The maximum increase in absorbance per min during coagulation was used as a measure of coagulant activity. The method was applied to normal platelet-rich plasma samples, and the effect of changing the concentration of plasma, thromboplastin, fibrinogen, and heparin was studied.plasma coagulant activity ; extrinsic pathway ; thromboplastin ; fibrinogen ; heparin Tests commonly used for the determination of plasma coagulant activity depend mainly on measuring the reaction velocity, or measuring the yield of coagulation activity. The determination of the clotting time, as a measure of reaction velocity is based on the relationship between the clotting time and the coagulant activity, which is a complex relationship due to the sequential nature of the coagulation reactions (Bloom and Thomas 1981). A plot of the clotting time against coagulant activity yields a hyperbola on rectangular coordinates, where small differences in clotting time represent major differences in activity, when the clotting times are short, and represent minor differences in activity, when the clotting times are long. A standard curve must also be used with each test to convert the clotting times to units or percentages of maximum activity (Williams et al. 1977).The determination of the product of coagulation activity usually adds one more stage to the test. For instance, thrombin formed during clotting is determined in the two stage prothrombin time by its ability to clot exogenous fibrinogen (Ware and Seegers 1949). The prothrombinase formed in the first stage of the thromboplastin generation test is determined in the second stage by its ability to clot substrate plasma (Brown 1984). Recently, thrombin generated during coagulation was determined by measuring the activity of thrombin eluted from the clot. This amount was found to be proportional to the total amount