A method of fractionation is described which permits the separation of 3 subcellular fractions from white matter a myelin fraction, a microsomal fraction, and a residue. The electron microscopic control of these fractions showed that the myelin and the microsomal ones are rather homogeneous, with slight contamination. The residue is a mixed fraction containing cyto-plasmic debris, distorted nuclei, mitochondria, erythrocytes, and some myelin lamellae. The meylin fraction was further purified by osmotic shock, which removes most of the axoplasm. At similar dose levels, the myelin fraction is 10 times more active than white matter in producing a half-maximal disease and is at least 13 times more potent than the residue. The myelin fraction obtained is an excellent material from which the isolation and purification of the antigen (or antigens) responsible for the production of E.A.E. can be studied, as well as for the chemical analysis of the myelin.