Methyl glucose lipopolysaccharides (MGLP) were first isolated in 1964 from a fast growing, nonpathogenic mycobacterial strain,Mycobacterium smegmatis. Their complete structure was achieved in 1982 by Forsberget al.2It was established that theM. smegmatisMGLP heterogeneity arises from the acyl appendages borne by the polysaccharidic core. In the present study, we report the occurrence of MGLP from a slow growing strainM. xenopiand the structure of the deacylated derivatives (MGP). A new analytical strategy, based on the use of High pH Anion Exchange liquid Chromatography (HPAEC) and Liquid Secondary Ion Mass Spectrometry (LSIMS) was successfully developed. Thanks to HPAEC, the MGP mixture was fractionated and from LSIMS data, it was clearly established that the heterogeneity of the MGP polysaccharidic core arises from the number of glycosyl and methoxyl units.