Galactoglucomannan (GGM) from cultures of Nicotiana plumbaginifolia has Man:Glc:Gal:Ara:Xyl in 1.0:1.1:1.0:0.1:0.04 ratio. Linkage analysis contained 4- and 4,6-Manp, 4-Glcp, terminal Galp and 2-Galp, small amounts and terminal Arap and terminal Xylp, and 0.03 mol acetyl per mol of glucosyl residue. Treatment with α- and β-d-galactosidases showed that the majority of the side-chains were either single Galp-α-(1 → residues or the disaccharide Galp-β-(1 → 2)-Galp-α-(1 → linked to O-6 of the 4-Manp residues of the glucomannan backbone. Analysis of the oligosaccharides generated by endo-(1 → 4)-β-mannanase digestion confirmed that the GGM comprises a backbone of predominantly alternating → 4)-d-Manp-β-(1 → and → 4)-d-Glc p-β-(1 → branched at O-6 of 65% of the 4-Manp residues. The major oligosaccharide identified was d-Glcp-β-(1 → 4)-[d-Galp-β-(1 → 2)-d-Galp-α-(1 → 6)]-d-Manp-β-(1 → (27%), and most of the other oligosaccharides produced in significant quantities were based on this structure.