A monoclonal antibody which reacts with prothrombin forming a complex with phosphatidylserine but not with prothrombin forming no complex with phosphatidylserine; and a method of assaying an activity of a test plasma of inhibiting blood coagulation which involves: (1) the step of assaying the inhibitory activity of the test plasma on blood coagulation reaction and its inhibitory activity on the blood coagulation reaction; and (2) the step of comparing the inhibitory activity of the test plasma with the inhibitory activity of the antibody.