Endohydrolysis of the di-N-acetylchitobiosyl unit in high-mannose glycopeptides and glycoproteins. Complex biantennary glycans are the preferred substrates. Tri- and tetraantennary glycans are not hydrolyzed, and high mannose glycans are very poor substrates. Endohydrolysis of the N,N'-diacetylchitobiosyl unit in high-mannose glycopeptides and glycoproteins containing the -[Man(GlcNAc)2]Asn- structure. One N-acetyl-D-glucosamine residue remains attached to the protein, the rest of the oligosaccharide is released intact. Monomer. Carbohydrates at Ser-73, Ser-89 and Ser-143 consist of (2-OMe)Man1-4GlcNAcU1-4GlcU1-4Glc1-4(2-OMe)GlcU1-4[(2-OMe)Rham1-2]Man. Belongs to the glycosyl hydrolase 18 family.