We have characterized the structure of a sulfated d-galactan from the red algae Botryocladia occidentalis. The following repeating structure (-4-alpha-d-Galp-1-->3-beta-d-Galp-1-->) was found for this polysaccharide, but with a variable sulfation pattern. Clearly one-third of the total alpha-units are 2,3-di-O-sulfated and another one-third are 2-O-sulfated. The algal sulfated d-galactan has a potent anticoagulant activity (similar potency as unfractionated heparin) due to enhanced inhibition of thrombin and factor Xa by antithrombin and/or heparin cofactor II. We also extended the experiments to several sulfated polysaccharides from marine invertebrates with simple structures, composed of a single repeating structure. A 2-O- or 3-O-sulfated l-galactan (as well as a 2-O-sulfated l-fucan) has a weak anticoagulant action when compared with the potent action of the algal sulfated d-galactan. Possibly, the addition of two sulfate esters to a single alpha-galactose residue has an "amplifying effect" on the anticoagulant action, which cannot be totally ascribed to the increased charge density of the polymer. These results indicate that the wide diversity of polysaccharides from marine alga and invertebrates is a useful tool to elucidate structure/anticoagulant activity relationships.