The specific capsular polysaccharide of Streptococcus pneumoniae type 15A (American type 30) is composed of D-galactose (three parts), D-glucose (one part), 2-acetamido-2-deoxy-D-glucose (one part), phosphate (one part), and glycerol (one part). Hydrolysis, periodate oxidation, methylation, optical rotation, and nuclear magnetic resonance studies showed that the polysaccharide is a high molecular weight linear polymer of a pentasaccharide repeating unit having the structure: (formula: see text)