Subtle modifications of N-linked oligosaccharides are known to mediate important biological functions. However, many of these modifications are destroyed by chemical treatments used to release oligosaccharides from the proteins. Therefore, enzymatic release of the intact, modified oligosaccharides is preferred. We have used a mixture of endoglycosidase F and Peptide: N-glycosidase F to release most, if not all, of the [2-3H]Man-labelled N-linked oligosaccharides from a wide variety of mammalian cell lines. This approach combined with biosynthetic radiolabelling could, therefore, be used to identify novel, rare or transient modifications of N-linked oligosaccharides. This treatment indeed released metabolically incorporated [35SO4] from each of 9 different cell lines demonstrating that sulfated N-linked oligosaccharides are much more common than previously believed.