A galactomannoglucan (GMG) with an estimated weight-average molar mass of 1.5×10 5 was obtained from an aqueous extract of the mesocarp of fruits of Arecastrum romanzoffianum (Cham.) Becc. by fractionation by Sephacryl S-300 HR and Sephadex G-25. Chemical and spectroscopic studies indicated that GMG has a chain of (1→4)-linked β-d-mannopyranosyl residues, a chain of (1→3)-linked β-d-galactopyranosyl residues, a chain of (1→4)-linked α-d-glucopyranosyl residues, repeating units of β-d-galactopyranosyl-(1→4)-β-d-mannopyranosyl and β-d-mannopyranosyl-(1→4)-α-d-glucopyranosyl and terminal residues of d-galactopyranosyl and d-glucopyranosyl which comprised galactose, mannose and glucose in the molar ratio of 10:37:53. The polysaccharide exhibited significant antiinflammatory activity against carrageenan-induced mouse paw oedema.