The influence of the degree of fucoidan macromolecules sulfation on their ability to induce an anticoagulant effect was studied. In in vitro experiments with the basic stain and in in vivo experiments on rabbits with performance of Quick's test, APTT, and spectrophotometric determination of antithrombin III (AT-III) activity a direct dependence of the intensity and duration of the anticoagulant effect on the degree of fucoidan sulfation was shown. The degree of sulfation was increased by alkaline treatment and anion-exchange chromatography of the compound. The anticoagulant effect of the fucoidan fractions under study was realized practically without affecting AT-III activity. And this direct effect of inhibition of the coagulation factors could be immediately arrested by intravenous injection of a neutralizing dose of the heparin antagonist VIM-DEMA.