An endo-beta-N-acetylglucosaminidase has been purified extensively from hen oviduct extracts and found to have a somewhat broader substrate specificity than the previously reported endoglycosidases from Dipolcoccus pneumoniae and Streptomyces plicatus (Arakawa, M., and Muramatsu, T. (1974) J. Biochem. (Tokyo) 76, 307-317; Tarentino, A. L., and Maley, F. (1975) Biochem. Biophys. Res. Commun. 67, 455-462. The enzyme was shown to hydrolyze and di-N-acetylchitobiosyl bond in such compounds as Asn(GlcNAc)2(Man)3, Asn(GlcNAc)2(Man)5, Asn(GlcNAc)2(Man)6 and a glycopeptide from immunoglobulin M, (aa)x-Asn(GlcNAc)2(Man)3(Fuc)1. The capacity to hydrolyze the latter compound is a characteristic of the endoglycosidase from D. pneumoniae, but not that from S. plicatus. Competitive inhibition studies indicate that a single enzyme in the hen oviduct endoglycosidase preparation is probably responsible for hydrolyzing both fucose-containing the fucose-depleted substrates. The existence of an endo-beta-N-acetylglucosaminidase in animal tissue may explain why oligosaccharides with N-acetylglucosamine on the reducing terminus accumulate in certain lysosomal storage diseases.