Two cases of resistance to the heparin treatment, induced by large consumption of antithrombin III, are reported. Normally small, this consumption does not automatically call for the systematic dosage of Antithrombin III for each treatment using heparin. On the other hand, any clinical or biological resistance to higher doses of heparin, any personal or family history of thrombosis necessitate the administration of Antithrombin III. Where there is an acquired or congenital deficit, an injection of Antithrombin III corrects the disorder and reduces the risk of thrombosis. In the case of a heparin-antivitamin K relay, it is essential to achieve complete biological effectiveness of the antivitamins K to interrupt the administration of the heparin.