The specific capsular polysaccharide of Streptococcus pneumoniae serotype 23F (American type 23) is composed of a repeating tetrasaccharide unit containing D-glucose (one part), D-galactose (one part), L-rhamnose (two parts), glycerol (one part), and phosphate (one part). By composition analysis, optical rotation, partial hydrolysis, periodate oxidation, methylation, and high-resolution 1H and 13C nuclear magnetic resonance studies, the elucidated unambiguous structure was in agreement with our earlier proposal but is at variance with structures proposed later by other authors. The structure of the type 23F pneumococcal polysaccharide is (formula; see text).