The properties of the contact factors, factor XII, high molecular weight kininogen and prekallikrein are described as well as the abnormalities in the hereditary deficiencies of these proteins. The interactions of each of these proteins with the other as well as their regulation by plasma proteolytic inhibitors such as Cl inhibitor and antithrombin III are delineated. Biochemical techniques for measuring this system are discussed. Conditions associated with abnormal synthesis of these proteins are described. Diseases in which increased kinin formation has been documented as well as disorders where there is strong evidence for the activation of kallikrein are presented. Further knowledge of this system should increase our understanding of its pathophysiological alterations.