The HNK-1 glycan, sulfoÆ3GlcAb1Æ3Galb1Æ4GlcNAcb1ÆR, is highly expressed in neuronal cells and apparently plays critical roles in neuronal cell migration and axonal extension. The HNK-1 glycan synthesis is initiated by addition of b1,3-linked GlcA to N-acetyllactosamine followed by sulfation of C-3 position of GlcA. The cDNAs encoding b1,3-glucuronyltransferase (GlcAT-P) and HNK-1 sulfotransferase (HNK-1ST) have been recently cloned. Among various adhesion molecules, the neuronal adhesion molecule (NCAM) was shown to contain HNK-1 glycan on N-glycans. In the present study, we first demonstrated that NCAM also bears HNK-1 glycan attached to O-glycans when NCAM contains O-glycan attachment scaffold, muscle-specific domain (MSD) and is synthesized in the presence of core2 b1,6-N-acetylglucosaminyltransferase, GlcAT-P, and HNK-1ST. Structural analysis of the HNK-1 glycan revealed that the HNK-1 glycan is attached on core2 branched O-glycans, sulfoÆ3GlcAb1Æ3Galb1Æ4GlcNAcb1Æ6(Galb1Æ3)GalNAc. Using synthetic oligosaccharides as acceptors, we found that GlcAT-P and HNK-1ST almost equally act on oligosaccharides mimicking N- and O-glycans. By contrast, HNK-1 glycan is much more efficiently added to N-glycans than O-glycans when NCAM was used as an acceptor. These results are consistent with our results showing that HNK-1 glycan is minimally attached to O-glycans of NCAM in fetal brain, heart and the myoblast cell line, C2C12. These results combined together indicate that HNK-1 glycan can be synthesized on core2 branched O-glycans but that the HNK-1 glycan is preferentially added on N-glycans over O-glycans of NCAM probably because N-glycans are extended further than O-glycans attached to NCAM containing MSD.