Lipopolysaccharide (LPS) of Escherichia coli strain 2513 (R4 core-type) yielded after alkaline deacylation one major oligosaccharide by high-performance anion-exchange chromatography (HPAEC) which had a molecular mass of 2486.59 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. NMR-analyses identified the chemical structure as where l-alpha-d-Hep is l-glycero-alpha-d-manno-heptopyranose and Kdo is 3-deoxy-alpha-d-manno-oct-2-ulopyranosylonic acid and all hexoses are present as d-pyranoses. We have also isolated the complete core-oligosaccharides of E. coli F653 LPS for which only preliminary data were available and investigated the deacylated LPS by NMR and MS. The proposed structure determined previously by methylation analysis was confirmed and is shown below. In addition we have quantified the side-chain heptose substitution of the inner core with GlcpN ( approximately 30%) and confirmed that this sugar is only present when the phosphate at the second l,d-Hepp residue is absent.