To obtain a tool to study the structural characterization and the detection of mucin derived from the gastric gland mucous cells, we developed a monoclonal antibody, designated HIK1083, against mucin purified from rat gastric mucosa. In an ELISA, HIK1083 reacted strongly with the mucin purified from a deep layer of the corpus and antrum but only slightly reacted with that obtained from the surface mucosal layer. The reaction of mucin and HIK1083 was inhibited by the oligosaccharides obtained by the alkaline borohydride reduction of antigenic mucin. Two purified oligosaccharide alditols reacting with the monoclonal antibody obtained from the antigenic mucin had one and two peripheral α-linked N-acetylglucosamine residues, respectively, according to the evidence from NMR spectroscopy. Moreover, among the commercially available p-nitrophenyl derivatives of monosaccharides, only p-nitrophenyl-N-acetyl-α-d-glucosaminide inhibited the reaction of this monoclonal antibody and the antigenic mucin in a concentration-dependent manner. These results, as well as the immunohistochemical observations, indicate that α-linked N-acetylglucosamine residues are specifically attached to the peripheral region of the carbohydrate moiety of the mucin synthesized in and secreted from the gastric-gland-type cells, and indicate that the monoclonal antibody HIK1083 recognizes this structure.