Mucin, a major component of mucus, is a highly Oâglycosylated, highâmolecularâmass glycoprotein extensively involved in the physiology of gastrointestinal mucosa. To detect and characterize mucins derived from siteâspecific mucous cells, we developed a monoclonal antibody, designated PGM34, by immunizing a mouse with purified pig gastric mucin. The reactivity of PGM34 with mucin was inhibited by periodate treatment of the mucin, but not by trypsin digestion. This suggests that PGM34 recognizes the carbohydrate portion of mucin. To determine the epitope, oligosaccharideâalditols obtained from pig gastric mucin were fractionated by successive gelâfiltration, ionâexchange, and normalâphase HPLC, and tested for reactivity with PGM34. Two purified oligosaccharideâalditols that reacted with PGM34 were obtained. Their structures were determined by NMR spectroscopy as Fucα1â2GalÎČ1â4GlcNAc(6SO3H)ÎČ1â6(Fucα1â2GalÎČ1â3)GalNAcâol and Fucα1â2GalÎČ1â4GlcNAc(6SO3H)ÎČ1â6(GalÎČ1â3)GalNAcâol. None of the defucosylated or desulfated forms of these oligosaccharides reacted with PGM34. Thus, the epitope of PGM34 was determined as the Fucα1â2GalÎČ1â4GlcNAc(6SO3H)ÎČâ sequence. Immunohistochemical examination of rat gastrointestinal tract showed that PGM34 stained surface mucous cells close to the generative cell zone in the gastric fundus and goblet cells in the small intestine, but only slightly stained antral mucous cells in the stomach. These data, taken together, show that PGM34 is a very useful tool for elucidating the role of mucins with characteristic sulfated oligosaccharides.