Brain slices from actively myelinating (26-28 days) quaking and normal littermates were dual-labeled with radioactive mannose and fucose for 2 h. Following the incubation myelin was isolated by sucrose density gradient centrifugation and the incorporation of sugars into the major myelinassociated glycoprotein (MAG) determined. The incorporation of mannose (an internal monosaccharide) and fucose (a terminal monosaccharide) was impaired in quaking by approximately 70 and 83% respectively as compared to control. The mannose/fucose ratio in quaking myelin was approximately 70% higher than in control. The results indicate an abnormal processing of the N-linked oligosaccharide moiety of MAG in quaking oligodendrocytes.