Extraction of the green seaweed Chaetomorpha anteninna with water, followed by fractionation using the copper complex, gave a sulphated heteropolysaccharide, [alpha]D27 +84 degrees, that contained arabinose (57.7%), galactose (38.5%), rhamnose (3.8%), and sulphate (11.9%). Application of methylation analysis, periodate oxidation, and partial acid hydrolysis indicated a branched structure with 4-linked arabinose, 4- and 3-linked galactose, and 4-linked rhamnose residues. Both the arabinose and galactose residues carried branches. Desulphation and analysis of the polymeric product showed that sulphate groups were located at C-2 of some of the interior arabinose residues, C-4 of some of the non-reducing-end arabisoyl groups, and C-3 of the non-reducing-end galactosyl groups.