We examined the distribution of myelin antigens recognized by monoclonal antibodies (mAbs) 01 and 04 in the developing ventral white matter of the cervical spinal cord of the rat using immunogold-labeled ultrathin cryosections. From the beginning of myelination after birth to multilamellar myelin in adult animals, we observed colocalization of 04 and 01 label in myelin. In the oligodendrocyte soma, immunolabel was found primarily over Golgi cisternae. In the oligodendrocyte processes, immunolabeling was also found in the cytoplasm and along the plasmalemma. More cytoplasmic 04 and 01 label was found in the external loop of myelin than in the internal loop. The amount of 01 and 04 label increased over compact myelin in proportion to the number of lamellae, but the label density per unit length of membrane remained approximately the same in compact myelin as in oligodendrocyte plasmalemma. We did not see a concentration gradient for either 04 or 01 label across, or along multilamellar myelin sheaths.