The structure of the Streptococcus mutans serotype e polysaccharide was studied in order to determine the chemical basis of the immunological cross-reactions observed between it and the streptococcal group E polysaccharide. The chemical structure was established using methylation analysis, periodate oxidation, partial methanolysis and 13C NMR spectroscopy. The polysaccharide was found to consist of a polyrhamnose backbone of alternating 2- and 3-linked α-l-rhamnose units and sidechain β-d-glucosyl units linked to the 2-position of rhamnose units in the backbone. This structure of the oligosaccharide repeating unit of the S. mutans serotype e polysaccharide was identical to that of the group-specific polysaccharide of group E Streptococcus. Possible explanations for the previously reported differences in these two polysaccharides are discussed. © 1986.