The purification and characterization of a pectic polymer, rhamnogalacturonan I, present in the primary cell walls of dicots is described. Rhamnogalacturonan I accounts for approximately 7% of the mass of the walls isolated from suspension-cultured sycamore cells. As purified, rhamnogalacturonan I has a molecular weight of approximately 200,000 and is composed primarily of l-rhamnosyl, d-galacturonosyl, l-arabinosyl, and d-galactosyl residues. The backbone of rhamnogalacturonan I is thought to be composed predominantly of d-galacturonosyl and l-rhamnosyl residues in a ratio of approximately 2:1. About half of the l-rhamnosyl residues are 2-linked and are glycosidically attached to C4 of a d-galacturonosyl residue. The other half of the l-rhamnosyl residues are 2,4-linked and have a d-galacturonosyl residue glycosidically attached at C2. Sidechains averaging 6 residues in length are attached to C4 of the l-rhamnosyl residues. There are many different sidechains, containing variously linked l-arabinosyl, and/or d-galactosyl residues.