Basic information concerning the molecular organization of the myelin membrane is an intrinsic requirement for understanding the neurochemical events leading to myelination, as well as the potential mechanism of demyelination that might exist at the molecular level for a variety of neurological diseases. The application of chemical, enzymatic, fluorescent, and immunological membrane probes has contributed significantly to this end, although the diverse structural complexity of the myelin sheath has permitted only a rudimentary understanding of its molecular organization. Nevertheless, compelling evidence is accumulating which suggests that components of myelin are asymmetrically distributed in the membrane. Such membrane asymmetry should not only provide important clues to the mechanisms of membrane assembly in the process of myelination, but should also serve as a paradigm for potential functional asymmetry of the individual components at the molecular level. One particularly useful membrane probe is galactose oxidase which has the capacity for identifying surface galactose residues in both glycoproteins and glycolipids on the external surface of the myelin sheath. The identification of these surface components on the myelin sheath is of primary importance since such components might be more readily susceptible to immunological damage or act as a viral receptor which ultimately might lead to demyelination.