The structure of the polysaccharide chain of Shigella boydii type 2 lipopolysaccharide was established using mainly 13C-n.m.r. spectroscopy, partial hydrolysis, Smith degradation, and methylation analysis. The repeating unit of the polysaccharide was concluded to be a branched hexasaccharide, as follows: (formula in text). Acetaldehyde was detected in the hyrolysate of the lipopolysaccharide, but no evidence was obtained to indicate that acetaldehyde is located in the polysaccharide moiety.