An l-arabino- d-galactan and an l-arabino- d-galactan-containing proteoglycan were isolated from hot phosphate-buffered saline extracts of radish seeds by ethanol fractionation, ion-exchange chromatography, and gel filtration, and found homogeneous by ultracentrifuge analysis and high-voltage electrophoresis. The proteoglycan consisted of 86% of a polysacchraide component containing β- l-arabinose and d-galactose as major sugar constituents, together with small proportions of d-xylose, d-glucose, and uronic acids, and 9% of a hydroxyproline-containing protein. Methylation analysis, periodate oxidation, and enzymic degradations indicated a backbone chain of (1å3)-linked β- dgalactosyl residues with side chains at O-6 of (1å6)-linked β- d-galactosyl residues and uronosyl groups. The α- l-arabinofuranosyl residues were located mainly in the outer regions as nonreducing groups, as well as O-2- or -5-linked inner chain residues, and O-2,5- or -3,5-linked branching residues. Reductive, alkaline degradation of the proteoglycan indicated that the polysaccharide chains were partly linked through O-glycosyl linkages to the threonine residues of the polypeptide chains. The proteoglycans from radish leaves and seeds appeared to share common antigenic determinant(s). The radish-seed arabinogalactan had a high content (81%) of l-arabinose and its basic structure seemed to be similar to that of the polysaccharide component of the proteoglycan.