Protein C is a vitamin K dependent protein that is present in plasma as a zymogen. Protein C is activated in vivo by thrombin in the presence of thrombomodulin. Protein C can be activated in vitro by a protein fraction derived from the venom of the copperhead snake Agkistrodon contortrix contortrix.

Deficiency of Protein C is associated with recurrent venous thrombosis, especially in young adults. Acquired deficiencies of Protein C are associated with hepatic disorders, oral anticoagulant therapy, and disseminated intravascular coagulation.