A monoclonal antibody having inhibitory activity on blood coagulation reactions, which reacts with prothrombin forming a complex with phosphatidylserine but does not react with prothrombin not forming a complex with phosphatidylserine; and a method for measuring inhibitory activity of test plasma on blood coagulation reactions, which comprises the steps of (1) measuring inhibitory activity of the test plasma on the blood coagulation reactions and inhibitory activity of this on the blood coagulation reactions, and (2) comparing the inhibitory activity of the test plasma with the inhibitory activity of this antibody.