Protein C is responsible for the rapid inactivation of factor Va following blood clotting in vitro.

When whole blood is allowed to clot in vitro, factor V is rapidly activated to factor Va which is subsequently inactivated. We developed two monoclonal anti-protein C antibodies, one of which inhibits protein C activation and the other inhibits protein C activity. The addition of either antibody to blood before clotting in vitro significantly inhibited the inactivation of factor Va, confirming the essential role of protein C in mediating the rapid inactivation of factor Va.