Rhamnogalacturonan II (RG-II) is a structurally complex pectic polysaccharide that had previously been isolated from the primary cell-walls of a dicot (sycamore, Acer pseudoplatanus) and a gymnosperm (Douglas fir, Pseudotsuga menziesii). Herein are described the isolation and structural characterization of RG-II from the primary cell-walls of a suspension-cultured monocot (rice, Oryza sativa). The glycosyl-linkage composition of rice RG-II was similar to that of sycamore and Douglas fir RG-II. Partial hydrolysis with acid was used to release, from rice RG-II, two disaccharides, a heptasaccharide, and an octasaccharide; all of these oligosaccharides had previously been shown to be components of sycamore RG-II. The rice RG-II disaccharides characterized were α-rhamnosyl-(1→5)-3-deoxy- d- manno-2-octulosonic acid (KDO) and β-arabinosyl-(1→5)-3-deoxy- d- lyxo-2-heptulosaric acid (DHA). The rice RG-II heptasaccharide contained, inter alia, aceric acid (3- C-carboxy-5-deoxy- l-xylose) and 2- O-methylfucose, and was shown to have the same residue sequence and same points of attachment of glycosyl linkages (except for that to aceric acid, which was not determined) as its sycamore counterpart. Finally, an octasaccharide containing, inter alia, terminal 2- O-methyl-xylose, 2-linked glucuronic acid, and 3,4-linked fucose was isolated after partial hydrolysis of rice RG-II with acid. The same octasaccharide had been postulated to be a component of sycamore RG-II; conclusive evidence of its existence as a component of rice RG-II was obtained by using fast-atom-bombardment mass spectrometric analysis. The results indicated that RG-II from the primary cell-walls of a monocot is indistinguishable from the previously characterized RG-II isolated from a dicot.