The specific capsular antigen ofStreptococcuspneumoniaeserotype 35B was shown by a combination of 2D NMR methods and mass spectrometric and classical carbohydrate chemical techniques to be a high molecular weight polymer containingD-galactose,D-glucose, 2-acetamido-2-deoxy-D-galactose, and ribitol (2:1:1:1). The pentasaccharide repeating unit is polymerized through phosphate diester linkages to give the structure,Seventy percent of the β-D-Galfresidues glycosidically linked to the ribitol units carry anO-acetyl substituent.Keywords:Streptococcuspneumoniae, capsular antigen, polysaccharide structure.