On graded hydrolysis and Smith degradation, the O-somatic polysaccharide isolated from Shigella dysenteriae type 10 bacteria yielded four oligosaccharides which were characterized by methylation studies. Immunochemical studies using constituent monosaccharides and the oligosaccharides indicated that the sugar grouping →3)-ManNAc-(1→3)-Rha-(1→4)-GlcNAc-(1→ was the immunodominant part in the polysaccharide molecule, and that 2-acetamido-2-deoxy-d-mannose contributed maximally to the immunological specificity of the macromolecule. © 1983.