We present a defined chemical method for the isolation of the oligosaccharide linking the protein to the disaccharide units of keratan sulfate from bovine corneas. 1) Hydrazinolysis removes the amino acids from the peptido keratan sulfate, and leads to deacetylation of the glucosamine residues but does not split off the sulfate groups. 2) NaB3H4 reduction selectively labels the reducing terminal glucosamine of the chain by converting it to [3H]glucosaminitol. 3) Nitrous acid deamination splits the glycosidic bonds of glucosamine and converts this sugar into 2,5-anhydromannose but also leads to several derivatives of the free terminal [3H]glucosaminitol. 4) Na12CN treatment stabilizes the reactive 2,5-anhydromannose and the terminal compounds containing aldehyde groups in a cyanhydrin reaction. 5) The oligosaccharide structure between the glucosamine residue at the reducing end and the first glucosamine of the disaccharide chain is not degraded by this procedure and is obtained intact and in labelled form. The data so far obtained on this part of the cornea keratan sulfate show that it is heterogeneous and contains besides the terminal glucosamine, mannose and fucose in a similar ratio as found in undegraded keratan sulfate. The predominant compound probably contains three neutral sugar residues.