A facile synthesis of oligosaccharide-thiazoline derivatives of N-glycans as a novel class of inhibitors for endo-beta-N-acetylglucosaminidases was described. It was found that the external sugar residues on the N-glycan core could enhance the inhibitory potency. While the Manbeta1,4GlcNAc- and Man3GlcNAc-thiazolines were only moderate inhibitors, the large Man9GlcNAc-thiazoline demonstrated potent inhibitory activity, with an IC(50) of 0.22 and 0.42 microM against the Arthrobacter enzyme (Endo-A) and the human endo-beta-N-acetylglycosaminidase (hENGase), respectively. It was also observed that the oligosaccharide thiazolines could differentially inhibit endo-beta-N-acetylglucosaminidases from different sources. These oligosaccharide thiazolines represent the first class of endo-beta-N-acetylglucosaminidase inhibitors.