Abstract A galactoglucomannan (GGM) with an estimated weight-average molar mass of 3.5 × 10 5 was obtained from an aqueous extract of the mesocarp of fruits of Acrocomia aculeata (Jacq.) Lodd. by fractionation by Sephacryl S-300 HR and Sephadex G-25. Chemical and spectroscopic studies indicated that GGM has a main chain of (1 → 4)-linked β- d-mannopyranosyl residues attached to an initial chain of (1 → 3)-linked β- d-galactopyranosyl residues and a terminal chain of (1 → 4)-linked α- d-glucopyranosyl residues which comprised galactose, glucose and mannose in the molar ratio of 18:22:60. The adjuvant potential of the polysaccharide on the cellular immune response against ovalbumin antigen was investigated using in vivo assays.