Fucans extracted from brown algae exhibit anticoagulant properties, as demonstrated by coagulation assays and kinetic study of the inactivation of thrombin in the presence of antithrombin III. These derivatives exert a direct antithrombic activity by an antithrombin III-independent pathway. Besides, their anticoagulant activity is not mediated by a potency to inhibit factor Xa. Relatively low-molecular-mass fractions (50,000 Daltons) prepared by chromatographic fractionation of the crude fucans also exhibit anticoagulant activity. They may prove useful as anticoagulant drugs.