SUMMARY: A polysaccharide constituted primarily of a unit of N-acetylneuraminic acid, two of glucosamine and one of an acid labile nitrogenous component was extracted from Citrobacter freundii O5:H30. The material after purification gave a single peak in the ultracentrifuge. N-acetylneuraminic acid and 4-oxo-norleucine were isolated from hydrolysates of the polysaccharide. A similarly constituted polysaccharide was obtained from Salmonella dahlem. C. freundii O5:H30 and S. dahlem were shown previously to be serologically related to one another and the results of the present investigations indicate that a chemical relationship also exists between these micro-organisms. Moreover, it is suggestive that the serologically related S. djkarta which contains neuraminic acid is related chemically to S. dahlem and C. freundii O5:H30. In sum, it is concluded that derivatives of neuraminic acid exist in association with other amino sugars and amino acids in mucopolysaccharides of bacterial origin as well as in those derived from mammalian origin.