The brown seaweed Sargassum stenophyllum biosynthesizes two different sets of fucoidans. One of them is characterized by higher percentages of glucuronic acid and fewer sulfate groups, which are situated on different sugar units. alpha-L-Fucose was the major component but other sugars like beta-D-galactose, beta-D-mannose, alpha-D-glucuronic acid, alpha-D-glucose and beta-D-xylose were also in substantial amounts. Fucoidans from the other set contain small amounts of alpha-D-glucuronic acid and high percentages of sulfate groups, which are concentrated on the fucose residues, with only fucose and galactose as major components. Structural studies of one fucoidan from each set suggest that these products have a general basic structure that has a formal resemblance to that of the fucosylated chondroitin sulfates from the body wall of sea cucumbers, namely, a linear core (formed mainly by (1-->6)-beta-D-galactose and/or (1-->2)-beta-D-mannose units) with branched chains of 'fucans' (formed by (1-->3) and/or (1-->4)-alpha-L-fucose, (1-->4)-alpha-D-glucuronic acid, terminal beta-D-xylose and, sometimes, (1-->4)-alpha-D-glucose). In fucoidans from the second set, the 'core' is reduced to short galactan chains.