termination of the reaction thrombin thrombomodulin complex snake venom and others may be mentioned as the pc activator but use of thrombin thrombomodulin complex is preferable in terms of physiologically natural conditions it may be added in the form of complex or thrombin and thrombomodulin may be added separately since activation of pc requires the presence of ca 2 ions it is preferable to add these ions to the buffer solution or to the solution of pc activator in an amount that will give optimum concentration in the pc activation system thrombin added to activate pc also acts upon the synthetic peptide substrate and hence interferes with the measurement of pc activity it is therefore necessary to add an inhibitor against thrombin after pc has been activated this inhibitor must preferably be such that it inhibits the action of thrombin and shows no effect upon pca antithrombin iii may be used for this purpose in actual practice it is preferable to use heparin in combination because it markedly accelerates the action of antithrombin iii however addition of antithrombin iii alone cannot inhibit the activity of the other interfering substances contained in the plasma in fact measurement of pc activity by the use of a synthetic substrate in combination with antithrombin iii heparin alone gave.