A mucopolysaccharide was isolated from mussel broth. After sulphation, an electrophoretically homogenous product was obtained with a molecular weight of approximately 40 000 daltons and a sulphur content of about 12%. The sulphated polysaccharide (S-Lim) displayed an anticoagulant activity in a thrombin test system with human plasma. Unlike heparin, the anticoagulant effect of S-Lim was observed also in a thrombin-fibrinogen clotting system in the absence of AT III. Complete inhibition of the effect of 1.0 mg S-Lim was achieved with 1.5 mg protamine. In an activated partial thromboplastin time test system the anticoagulant activity of 1.0 mg S-Lim corresponded to about 40 iu of heparin. S-Lim was also found to inhibit thrombin-induced platelet aggregation. Furthermore, S-Lim inhibited the thrombin-dependent activation of factor XIII in human plasma. S-Lim did not affect various tests systems measuring factor Xa activity. It is concluded that this new sulphated mucopolysaccharide acts as a pure antithrombin with a potency corresponding to 40--50 iu heparin/mg S-Lim. In contrast to heparin and other heparinoids it does not require the presence of AT III.