Natural compounds offer interesting pharmacological perspectives for antiviral drug development with regard to broad-spectrum antiviral properties and novel modes of action. In this study, we have analyzed polysaccharide fractions isolated from Grateloupia indica. The crude water extract (GiWE) as well as one fraction (F3) obtained by anion exchange chromatography had potent anti-HSV activity. Their inhibitory concentration 50% (IC50) values (0.12-1.06 microg/ml) were much lower than cytotoxic concentration 50% values (>850 microg/ml). These fractions, which were effective antiviral inhibitors if added only during the adsorption period, had very low anticoagulant activity. Furthermore, they had no direct inactivating effect on virions in a virucidal assay. Chemical, chromatographic and spectroscopic methods showed that the active polysaccharide, which has an apparent molecular mass of 60 kDa and negative specific rotation [alpha]D(32) -16 degrees (c 0.2, H2O), contains alpha-(1-->4)- and alpha-(1-->3)-linked galactopyranose residues. Sulfate groups, if present, are located mostly at C-2/6 of (1-->4)- and C-4/6 of (1-->3)-linked galactopyranosyl units, and are essential for the anti herpetic activity of this polymer.