Abstract Group A meningococcal polysaccharide was shown to be a homopolymer of d-mannosamine phosphate which is partially N- and O-acetylated. The principal glycosidic bond in this polysaccharide seems to be a 1—6 phosphodiester bond involving the hemiacetal group of carbon 1 and the alcohol group of carbon 6 of the d-mannosamine. There is also evidence to suggest that other glycosidic linkages exist. The exact nature and the position of these linkages are not yet known. The average chain length of this polysaccharide determined by reducing group analysis is 93 residues. The molecular weight of the polymer is in excess of 100,000 which implies that there are several chains in the A polysaccharide. A procedure has been developed for the determination of reducing group present in an oligosaccharide. This procedure was developed based on the observation that, when a hexosamine such as mannosamine or glucosamine was first treated with sodium borohydride to convert the carbonyl function into an alcohol, the resulting aminohexitol remained qualitatively and quantitatively unchanged by subsequent treatment with the oxidizing agent K3Fe(CN)6 at 100° for 10 min, whereas hexosamine similarly treated with K3Fe-(CN)6 was quantitatively destroyed. When dealing with an oligosaccharide consisting of hexosamine which had been treated with sodium borohydride, the ratio of the values of two analyses, one performed before oxidation with K3Fe(CN)6 and the other after, yields the fraction of hexosamine with its reducing group free.