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The O antigen polysaccharide of Salmonella montevideo was isolated from a core-defective mutant by the phenol/water procedure, and was suspected to contain phosphomonester and cyclic phosphodiester at its reducing end in anology to the O hapten from Salmonella typhimurium (Kent and Obsborn, 1968. Therefore, it was chromatographed on a DEAE-cellulose column. Whereas one part eluted with water the other part of the polysaccharide could only be eluted with buffer. Both fractions were further purified on Sephadex G100 and contained mannose, glucose, N-acetylglucosamine and phosphate in a molar ratio of 4:1:1: less than 0.1. In order to specifically label the reducing end phosphate was removed enzymatically, or the presumed cyclic diester was cleaved by mild hydrolysis, and the fractions were reduced with sodium horo[3H]hydride. Both fractions yield mainly [3H]glycerol after hydrolysis and paper chromatogaphy. In addition, [3H]mannitol and [H]monohydroxyacetone could be identified by paper chromatography and were concluded to be the result of phosphate migration and beta-elimination reactions taking place during the isolation procedure and the various treatments prior to sodium boro[3H]hydride reduction. These findings in addition to periodate oxidation studies indicated that the O antigen polysaccharide of Salmonella montevideo had glyceraldehyde phosphate at its reducing end. From the incorporation of 3H into the polysaccharide the O antigen was calculated to consist of about 19 repeating units of 6 sugar residues each.