alpha1,4-N-acetylglucosaminyltransferase (alpha4GnT) is a glycosyltransferase that mediates transfer of GlcNAc to betaGal residues with alpha1,4-linkage, forming GlcNAcalpha1--> 4Galbeta-->R structures. In normal human tissues, glycoproteins having GlcNAcalpha1-->4Galbeta-->R structures at non-reducing terminals are exclusively limited to the mucins secreted from glandular mucous cells of gastric mucosa, Brunner's gland of duodenum, and accessory gland of pancreaticobiliary tract. Recently, we have isolated a cDNA encoding human alpha4GnT by expression cloning. Although alpha4GnT plays a key role in producing this unique glycan in vitro, the actual localization of alpha4GnT was not determined. In this study we examined the localization of alpha4GnT in various human tissues, including gastrointestinal mucosa, using a newly developed antibody against human alpha4GnT. The specificity of the antibody was confirmed by analyses of human gastric adenocarcinoma AGS cells transfected by alpha4GnT cDNA. Expression of alpha4GnT was largely associated with the Golgi region of mucous cells that produce the mucous glycoproteins having GlcNAcalpha1-->4Galbeta-->R, such as the glandular mucous cells of stomach and Brunner's gland. An immunoprecipitation experiment disclosed that two distinct mucin proteins, MUC5AC and MUC6 present in gastric mucin, carried the GlcNAcalpha1-->4Galbeta-->R structures. These results indicate that alpha4GnT is critical to form the mucous glycoproteins having GlcNAcalpha1-->4Galbeta-->R on MUC6 and MUC5AC in vivo.(J Histochem Cytochem 49:587-596, 2001)