Congenital antithrombin abnormality was found in several members of a French family. No history of thrombotic episodes was associated with this abnormality. Plasma antithrombin concentration as well as the rate of thrombin inactivation by defibrinated plasma in the absence of heparin were normal. However, the heparin cofactor activity was decreased by about 50% in plasma of affected patients. Accordingly, about half the amount of plasma antithrombin did not bind to gel bound heparin. Moreover the crossed immunoelectrophoretic pattern in the presence of heparin demonstrated two peaks of antithrombin, the slower one migrating as normal antithrombin when heparin was omitted from the first agarose gel. It was concluded that molecular alteration of the antithrombin molecule seemed to affect only the heparin binding site thus preventing from any rate enhancement of thrombin inactivation.