A set of Lipooligosaccharides (LOSs) has previously been characterized inM. gastri W471. The structure of the highly antigenic LOS (LOS-III) was elucidated and this molecule can unambiguously distinguishM. gastri from the opportunistic pathogenM. kansasii. In the present study, the structures of three otherM. gastri W471 LOSs were determined by one-dimensional¹H-NMR spectroscopy and gas liquid chromatography. They differ by the number of Xylp units and by the structure of the distal monosaccharide. The two dimensional (2D) NMR approach was successfully applied to the LOS antigen ofM. kansasii to locate the acetyl and acyl substituents and to determine the anomeric configuration of the -d-Fucp unit.