The structure of the antigenic O-polysaccharide (O-PS) produced by Escherichia coli serotype O:70 was determined by analysis of the chromatographically purified O-PS polymer prepared by mild hydrolysis of its aqueous phenol-extracted smooth-type somatic lipopolysaccharide. The O-PS is composed of D-glucose, D-galactose, D-fucose, 2-acetamido-2-deoxy-D-galactose, and 3-acetamido-3-deoxy-D-quinovose in a ratio of 1:1:1:1:1. From the use of DOC-PAGE, methylation, Smith-type periodate oxidation, and (1)H and (13)C NMR spectroscopy, including 2D experiments, the O-PS was shown to be a polymer of a branched repeating pentasaccharide unit having the structure: [structure: see the text]