Water-soluble arabinogalactan-proteins have been isolated from tissue-cultured rye grass endosperm cells and their culture medium by precipitation with the β-glucosyl Yariv artificial antigen. The intracellular and extracellular polymers are similar in composition and molecular size (apparent mol. wt 2.2 — 2.8 × 105). The protein portion represents up to 7% of the molecule and is rich in hydroxyproline, alanine and serine. The carbohydrate portion (84%) consists solely of arabinose (36%) and galactose (64%) and methylation analysis shows it to be a branched 1,3 : 1,6-galactan substituted by arabinofuranosyl residues. The carbohydrate compositions and methylation analyses for polymers precipitated by the β-glucosyl Yariv antigen from lima beans, cashew, tomato, silver beet and asparagus and an arabinogalactan-peptide from wheat endosperm are compared. As a group, they show homologies both in their peptide and polysaccharide portions. A histochemical method, based on precipitation with the β-glucosyl Yariv antigen, showed the binding polymers to be localized in discrete irregular vesicles in the tissue-cultured rye grass endosperm cells. Native rye grass endosperm cells show staining between starch granules, and staining is also seen in the aleurone cells of barley, wheat and rye grass.