Stable blood coagulation factor activated by contact with the subendothelial surface of an injured vessel. Along with prekallikrein, it serves as the contact factor that initiates the intrinsic pathway of blood coagulation. Kallikrein activates factor xii to XIIa. Deficiency of factor xii, also called the Hageman trait, leads to increased incidence of thromboembolic disease. Mutations in the gene for factor xii that appear to increase factor xii amidolytic activity are associated with hereditary angioedema type iii.