Mild acid degradation of the Pseudomonas fluorescens strain ATCC 49271 lipopolysaccharide resulted in a core oligosaccharide containing D-glucose, 2-acetamido-2-deoxy-D- glucose, 2-(L- alanylamino)-2-deoxy-D-galactose, 2-acetamido-2,6-dideoxy-D-glucose (QuiNAc), 2-acetamido- 2,6-dideoxy-L-galactose (FucNAc), L-glycero-D-manno-heptose (Hep), 3-deoxy-D- manno-octulosonic acid (Kdo, present in multiple forms), and 5-acetamidino-7-acetamido-3,5,7,9- tetradeoxy- L-glycero-D-galacto-nonulosonic acid (a di-N-acyl derivative of legionaminic acid, Non) as well as O-acetyl, O-carbamoyl, and phosphate groups, including triphosphate groups. The dephosphorylated (HF) decasaccharide and products of its partial and full O-deacylation were studied by methylation analysis, GLC-MS, and 1H NMR spectroscopy, including 1D NOE and 2D shift-correlated spectroscopy (COSY). The core oligosaccharide of P. fluorescens strain ATCC 49271 was found to be a decasaccharide (with partially degraded Kdo region) and one O-antigen repeating unit (di-N-acyllegionaminic acid, Non) attached. The following structure of the dephosphorylated core oligosaccharide was established: [sequence: see text]