The structure of the O-polysaccharide component of the lipopolysaccharide produced by Escherichia coli 0119 was determined by the use of methylation analysis, periodate oxidation, 1D and 2D nuclear magnetic resonance spectroscopy, and mass spectrometric methods. The O-polysaccharide was found to be a high molecular weight polymer of a repeating pentasaccharide unit composed of D-mannose, D-galactose, L-rhamnose, 2-acetamido-2-deoxy-D-glucose, and 2-acetamido-2,3-dideoxy-3-formamido-D-rhamnose residues (1:1:1:1:1) and had the structure: [formula: see text]