Mass spectrometry has been applied for a detailed structural characterization of glycosphingolipids. Methylated and methylated plus reduced glycolipids (amide groups of ceramide and amino sugars were reduced to the corresponding amines) were found the most useful derivatives. Molecular weight ions [M – 1] were not obtained for methylated lipids with more than four sugar units, but were easily obtained for reduced derivatives, so far with six sugars. Spectra of reduced derivatives showed very abundant ions containing the complete carbohydrate and the fatty acid, allowing a conclusion to be made concerning the carbohydrate composition (ratio of hexose and hexosamine and sialic acid). In addition, conclusive information was obtained for the carbohydrate sequence, including branching of the chain. By a combined use of both derivatives the long chain base and fatty acid composition was also evident. Spectra are discussed for a synthetic monoglycosylceramide, the Forssman glycolipid hapten (pentaglycosylceramide) and two gangliosides (a triglycosylceramide and a pentaglycosylceramide). The mass spectrum of the Forssman glycolipid provides the first conclusive evidence for two amino sugars in the terminal disaccharide of the molecule.