Myelin-associated glycoprotein (MAG) has potent neurite outgrowth inhibitory activity in vitro. To assess the importance of MAG in the neurite outgrowth inhibitory activity in CNS myelin, we used an in vitro bioassay to characterize neurite growth on CNS myelin derived from mice carrying a null mutation of the MAG gene. Myelin proteins from MAG-deficient mice inhibited neurite outgrowth to a similar degree to the wild-type CNS myelin. These results suggest that CNS myelin molecules other than MAG exert strong inhibitory effects on the growth of neurites.