Protein C, protein S, and antithrombin III are naturally occurring anticoagulant proteins. Activated protein C and protein S act by inhibiting the action of the cofactors factor Va and factor VIIIa. Antithrombin III inhibits the serine proteases (factors II, IX, X, XI, and XII); its anticoagulant action is dramatically enhanced by heparin. Deficiencies of these proteins are associated with thromboembolic disease. Antigen and activity assays practical for the clinical laboratory are available for protein C and antithrombin III. Protein S antigen assays are also available. Diagnosis of an anticoagulant protein deficiency has serious implications for a patient and his kindred, and should therefore only be made on the basis of repeated abnormal determinations of a single protein in an individual patient, or documentation of the same protein deficiency in other family members.