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The core 3 structure of the O-glycan, GlcNAc␤1-3GalNAc␣1-Ser͞ Thr, an important precursor in the biosynthesis of mucin-type glycoproteins, is synthesized by ␤1,3-N-acetylglucosaminyltransferase 6 (␤3Gn-T6; core 3 synthase). We generated an anti-␤3Gn-T6 mAb (G8-144 mAb) and performed immunohistochemical analyses. In normal stomach and colon, ␤3Gn-T6 was strongly expressed in the Golgi region of epithelia. In contrast, its expression was markedly down-regulated in gastric and colorectal carcinomas. Tissue specimens from a familial adenomatous polyposis patient showed a clear correlation between the down-regulation of ␤3Gn-T6 expression and the degree of dysplasia͞neoplasia. In vitro, the level of ␤3Gn-T6 transcript was increased according to the differentiation of Caco-2 cells. These results suggested that the expression of ␤3Gn-T6 is closely regulated during differentiation and dedifferentiation. ␤3Gn-T6 would be a useful marker for distinguishing between benign adenomas and premalignant lesions. HT1080 FP-10 cells stably transfected with the ␤3Gn-T6 gene showed a decrease in the core 1 structure, Gal␤1,3GalNAc␣1-Ser͞ Thr, probably due to competition between the core 1 synthase and core 3 synthase. The migration activity of the transfectants was markedly lower than that of mock transfectants in vitro, and lung metastasis after i.v. injection of the transfectants into nude mice was significantly suppressed. These findings indicated that the core structures of O-glycans are profoundly involved in the metastatic capacity of cancer cells.glycosyltransferase ͉ stomach ͉ familial adenomatous polyposis ͉ immunohistochemical analysis