In glycoproteomics, key structural issues, protein identification, locations of glycosylation sites, and the glycosylation site microheterogeneity, should be evaluated in a large number of glycoproteins. To this end, a simple and efficient method, utilizing hydrophilic binding of carbohydrate matrixes such as cellulose and Sepharose to oligosaccharides, was successfully applied to the isolation of glycopeptides from tryptic digests. Both peptide and oligosaccharide structures were elucidated by multiple-stage tandem mass spectrometry (MSn) of the ions generated by matrix-assisted laser desorption/ionization. The method was applied to the analysis of glycan heterogeneities at seven N-glycosylation sites in each of the plasma and cellular fibronectins. In addition, a new O-glycosylation site was identified at Thr279. As a field work using a mass spectrometric strategy including the glycopeptide analysis described above, we have launched a screening study of Congenital Disorders of Glycosylation (CDG). CDG is caused by a defect of one of the nearly one hundred glycosylation or glycosidic enzymes which are involved in the assembly or processing of the N-glycans in endoplasmic reticulum or Golgi apparatus, and thus forms a very large groups of monogenic disorders. Since most glycoproteins are affected, the symptoms and signs of this syndrome is too diverse to diagnose without detailed analysis of the molecular abnormality of glycoproteins. Our activity will delineate most clearly the functional role of glycoprotein glycans in biological systems.