N-Acetylglucosaminyltransferase III (GnT-III), which catalyzes the addition of the bisecting GlcNAc branch on N-glycans, is usually described as a metastasis suppressor. Overexpression of GnT-III inhibited migration in multiple types of tumor cells. However, these results seem controversial to the clinical observations for the increased expression of GnT-III in human hepatomas, glioma, and ovarian cancers. Here, we present evidence that these inconsistencies are mainly attributed to the different expression pattern of cell sialylation. In detail, we show that overexpression of GnT-III significantly inhibits ␣2,3-sialylation but not ␣2,6-sialylation. The migratory ability of cells without or with a low level of ␣2,6-sialylation is consistently suppressed after GnT-III overexpression. In contrast, the effects of GnT-III overexpression are variable in tumor cells that are highly ␣2,6-sialylated. Overexpression of GnT-III promotes the cell migration in glioma cells U-251 and hepatoma cells HepG2, although it has little influence in human breast cancer cell MDA-MB-231 and gastric cancer cell MKN-45. Interestingly, up-regulation of ␣2,6-sialylation by overexpressing -galactoside ␣2,6-sialyltranferase 1 in the ␣2,6-hyposialylated HeLa-S3 cells abolishes the anti-migratory effects of GnT-III. Conversely, depletion of ␣2,6-sialylation by knock-out of -galactoside ␣2,6-sialyltranferase 1 in ␣2,6-hypersialylated HepG2 cells endows GnT-III with the anti-migratory ability. Taken together, our data clearly demonstrate that high expression of ␣2,6-sialylation on the cell surface could affect the anti-migratory role of GnT-III, which provides an insight into the mechanistic roles of GnT-III in tumor metastasis.Alterations in N-linked glycosylation have been observed in various malignant diseases such as cancer. Certain glycan structures resulting from the dysregulated glycosyltransferases and glycosidases are well known tumor markers and closely associated with the malignant progression (1). As examples, the ␣1,6-fucosylated ␣-fetoprotein catalyzed by ␣1,6-fucosyltransferase has been developed as a biomarker for primary hepatoma by Abelev (2). An increase in 1,6-GlcNAc branching of N-glycans as a consequence of enhanced expression of N-acetylglucosaminyltransferase V (GnT-V) 2 is convincingly correlated with the increased frequency of tumor metastasis (3-6). N-Acetylglucosaminyltransferase III (GnT-III) is one more important glycosyltransferase that has been receiving much attention for its involvement in the biology of tumor (7,8). It catalyzes the transfer of N-acetylglucosamine (GlcNAc) in a 1,4 linkage to mannose on N-glycans forming a bisecting GlcNAc structure. This step plays critical roles in determining the structure of N-glycans because introduction of the bisecting GlcNAc residue could preclude further processing and elongation of N-glycans, such as the formation of 1,6-GlcNAc branching structures (9). Considering also that overexpression of GnT-III inhibited the metastasis of multiple types of carcinomas (10, 1...