Oligosaccharide chains fluorescence-labelled at their reducing end with 2-aminopyridine were analysed by electrospray ionization mass spectrometry (ESIMS). Protonated molecules of complex type oligosaccharides were detectable at a level of 1 pmol. Fragmentation at the glycosidic bonds occurred in the electrospray ion source when the voltage between the capillary nozzle and the first skimmer was elevated, referred to as "in-source fragmentation". Most fragment ions observed had retained the pyridylaminated (PA) site of the molecule and gave significant information on the saccharide sequence. In conjunction with the accumulating data of reversed-phase chromatography of PA oligosaccharides, ESIMS of PA oligosaccharide will be useful as a sensitive method in the structural study of oligosaccharides from various biological sources.