Catabolism of Asn-linked glycoproteins to monosaccharides and amino acids occurs in lysosomes. Break-down must be complete to avoid lysosomal storage diseases that occur when fragments as small as dimers are left undigested. Recent results have clarified several aspects of Asn-linked glycoprotein catabolism in mammals. First, degradation of the oligosaccharide portion is accomplished by exo-glycosidases, which act only from the nonreducing end of chains to release sugar monomers as products. In contrast, proteolysis can proceed from both end and internal points along the polypeptide to eventually yield free amino acids. A second important feature of the glycoprotein disassembly pathway is that the hydrolytic steps can be grouped into two sets of ordered reactions: I) stepwise hydrolysis of the major portion of the oligosaccharide chains by a set of exoglycosidases, and II) ordered disassembly of the protein and the oligosaccharide-to-protein linkage region. Process II can vary at a single reaction step de...