Lipopolysaccharide (LPS) ofEscherichia colistrain2513 (R4 core-type) yielded after alkaline deacylation one major oligosaccharide by high-performance anion-exchange chro-matography (HPAEC) which had a molecular mass of Da as determined by electrospray ionization mass spectrometry. This was in accordance with the calculated molecular mass of a tetraphosphorylated dodecasaccharide of the composition shown below.