The in vitro and in vivo pharmacological properties of two oversulfated dermatan sulfate (DS) derivatives, S-DS1 and S-DS2, containing 2 and 3.7 sulfate groups/disaccharide unit respectively were compared to those of the parent DS (1 sulfate group/disaccharide unit). In a purified system the ability of S-DS1 and of S-DS2 to catalyse thrombin inhibition by heparin cofactor II was increased by ten- and seventeen-fold respectively. These compounds also had more potent anticoagulant activities in the activated partial thromboplastin time and the thrombin clotting time assays. Plasma immunodepleted in antithrombin III, heparin cofactor II and both cofactors allowed it to be demonstrated that these enhanced anti-coagulant activities were partly (S-DS1) or totally (S-SD2) independent of any plasma cofactors. In spite of these enhanced anticoagulant activities in vitro the oversulfated derivatives did not exhibit an increased antithrombotic activity in a thromboplastin Wessler type model. Moreover, at the doses investigated, S-DS2 had no antithrombotic effect. The influence of oversulfation on the pharmacokinetic pattern of DS was also investigated. As reported for unfractionated heparin, the biological activities generated after IV injection of high doses of S-DS1 and S-DS2 disappeared according to a concave-convex pattern. This may result from the higher affinities of S-DS1 and of S-DS2 toward endothelial cells in comparison with that of DS.