Demyelination was complement dependent because heating the sera to 56°C abolished the demyelinating activity, which was restored by the addition of fresh guinea pig serum. With the use of immunofluorescent techniques, globulins in demyelinating sera were localized on myelin sheaths of cerebellar cultures, which was confirmed in a later study with immunoperoxidase methods. The inhibition of myelination was complement dependent and reversible, was specific for oligodendrocytes and central nervous system myelin, and was obtained at far lower concentrations than required for demyelination. Because of the last attribute, myelination inhibition appeared to the authors to be a more sensitive and reliable index of antimyelin activity than demyelination. Both central nervous system and peripheral nervous system myelin are formed by the spiral wrapping of cytoplasmic processes of the respective myelin forming cells around central or peripheral axons, followed by extrusion of the cytoplasm to allow apposition of the inner cytoplasmic membranes.