Anticoagulation effect of chitosan sulfate ester and some mechanisms of its influence on hemostasis were studied in vitro. The chitosan sulfate ester catalyzed more effectively inactivation of the enzymes involved in the inner pathway of blood coagulation affecting only slightly the activity of outer pathway factors. The ester, similarly to heparin, inhibited blood coagulation mainly via accelerated thrombin inactivation by means of blood plasma antithrombin III. Inhibition of the enzyme coagulation activity using heparin cofactor II occurred only at high concentrations of chitosan sulfate ester in blood plasma. Besides, the ester decreased the thrombin catalytic activity in reactions with natural and synthetic peptide substrates. Thus, anticoagulation effect of chitosan sulfate ester is similar to that of heparin.