The kinetics of the blood coagulation system have been formulated and an expression obtained for the “prothrombin time” in terms of the concentrations of the components of the system. A linear plot of data obtained from plasma dilution curves gives the values of the parameters of the system, and yields a mathematical method of comparing relative thromboplastin potencies. The analytical expressions given lead to the proper choice of thromboplastin potency and plasma dilution for minimum error in the clinical determination of prothrombin.