A large kindred from eastern Kentucky, with extensive history of recurrent venous thrombosis and pulmonary embolism, was studied. Low antithrombin III titers, ranging from 26% to 49% of normal values, were found in plasma of nine members in three consecutive generations; another five members, four of whom were not available for study, are suspected of having the biochemical defect. There was a good correlation between clinical symptoms and antithrombin III deficiency, although three of the younger members with the defect still remained free of thrombosis. In serum of the affected subjects antithrombin III was almost completely utilized, which indicates that stoichiometric binding to coagulation enzymes dominates under biological conditions. Antithrombin and antifactor Xa activities residing in the macroglobulin region of plasma and serum remained unchanged. The responsiveness to heparin in vitro and in vivo confirmed the evidence that antithrombin III is the sole blood component through which heparin exerts its anticoagulant effect. In five affected members therapy with oral anticoagulants increased very significantly the level of antithrombin III in plasma and contributed to a remarkable increase of residual antithrombin III in serum. This objective improvement after warfarin therapy may create significant difficulties in the laboratory diagnosis of antithrombin III deficiency.