Fucoidan, a complex sulfated polysaccharide containing substantial percentages of L-fucose and sulfate ester groups, is a constituent of brown algae and some marine invertebrates. Besides L-fucose, it frequently contains D-xylose, D-galactose, D-mannose and D-glucuronic acid. Fucoidan has no determined molecular formula, and its structure built up mainly of (1→02)-linked, 4-O-sulfated fucopyranose residues has been generally accepted for fucoidan from Laminaria Japonica/ Undaria Pinnatifida.