The native polysaccharide antigen isolated from type IV group B Streptococcus contains D-galactose, D-glucose, 2-acetamido-2-deoxy-D-glucose, and sialic acid in the molar ratio 2:2:1:1 and is composed of the following repeating unit: The structural analysis of this antigen was based on results obtained from methylation analysis, partial hydrolysis, nitrous acid deamination, and nuclear magnetic resonance spectroscopic techniques. Keywords: capsular polysaccharide, high resolution NMR spectroscopy, group B Streptococcus, polysaccharide structure, polysaccharide degradation.