Myelin-associated glycoprotein (MAG) is expressed in periaxonal membranes of myelinating glia where it is believed to function in glia–axon interactions by binding to a component of the axolemma. Experiments involving Western blot overlay and coimmunoprecipitation demonstrated that MAG binds to a phosphorylated neuronal isoform of microtubule-associated protein 1B (MAP1B) expressed in dorsal root ganglion neurons (DRGNs) and axolemma-enriched fractions from myelinated axons of brain, but not to the isoform of MAP1B expressed by glial cells. The expression of some MAP1B as a neuronal plasma membrane glycoprotein (Tanner, S.L., R. Franzen, H. Jaffe, and R.H. Quarles. 2000. J. Neurochem. 75:553–562.), further documented here by its immunostaining without cell permeabilization, is consistent with it being a binding partner for MAG on the axonal surface. Binding sites for a MAG-Fc chimera on DRGNs colocalized with MAP1B on neuronal varicosities, and MAG and MAP1B also colocalized in the periaxonal region of myelinated axons. In addition, expression of the phosphorylated isoform of MAP1B was increased significantly when DRGNs were cocultured with MAG-transfected COS cells. The interaction of MAG with MAP1B is relevant to the known role of MAG in affecting the cytoskeletal structure and stability of myelinated axons.