Lipopolysaccharide from pathogenic Yersinia pseudotuberculosis type III, strain 209, has been isolated and characterized. Lipopolysaccharide was shown to contain lipid A (30%), protein (4.4%), 2-keto-3-deoxyoctonate (7.8%) and a polysaccharide component (45%), which consisted of paratose, L-fucose, D-mannose, D-glucose, D-galactose (trace), L- and D-glycero-D-manno-heptose, 2-deoxy-2-amino-D-glucose and 2-deoxy-2-amino-D-galactose residues. Molecular -sieve chromatography on Sephadex G-50 afforded on O-specific polysaccharide, being hydrolysed to yield equimolar amounts of paratose, L-fucose, D-mannose and 2-deoxy-2-amino-D-galactose. The chemical repeating unit of the O-specific polysaccharide has been suggested on the basis of methylation studies and periodate oxidation data.