Phenolic glycolipids play a key role as an antigenic probe for serodiagnosis of some human pathogen mycobacterial infections. The lipidic part which corresponds to a phenolphthiocerol dimycocerosate molecule, and the presence of partial O-methylated sugars, confer a high hydrophobicity to this kind of molecule. Fast atom bombardment (FAB) mass spectrometric analysis with standard matrices such as glycerol or thioglycerol was unsuccessful. Using a new matrix--monobutyltriethylene glycol--FAB analysis allows molecular weight determination and partial structural elucidation of the saccharidic and the lipidic part of those compounds.