1. An arabinogalactan-peptide from wheat endosperm was studied by using physicochemical techniques and some aspects of its chemical structure were determined. 2. The arabinogalactan-peptide is a non-associating, polydisperse macromolecule ([unk]=22000) which exhibits only minor non-ideal effects in aqueous solution. 3. Examination of the products of partial acid hydrolysis of the polysaccharide component showed that arabinose is present in the alpha-l-arabinofuranosyl configuration, and i.r.-absorption spectroscopy and optical-rotation studies suggest that the d-galactopyranose residues are linked by glycosidic linkages in the beta-anomeric configuration. 4. The arabinogalactan is linked to a peptide which represents 8% (w/w) of the arabinogalactan-peptide and which may be present as a molecular core. Partial degradation of the polymer by successive treatment with oxalic acid and NaOH showed that the linkage between polysaccharide and peptide involves galactose and hydroxyproline residues and is glycosidic in nature. A tentative model is proposed for the structure of the wheat endosperm arabinogalactan-peptide. 5. The subcellular location and function of the arabinogalactan-peptide is discussed in relation to previous work with related molecules.