The amino acid sequences of the encephalitogenic basic protein, A1, from bovine and human myelin are similar, differing by only 11 residues. The sequence reveals that while basic residues are spread randomly over most of the polypeptide chain, several regions (8-10 residues) exist that are nonpolar in character. The bovine protein has 170 residues with molecular weight 18,400. The human protein, which has an additional His-Gly sequence, contains 172 residues. The major encephalitogenic determinant (tryptophan region) of the bovine protein differs from the human only by a lysine to arginine substitution. The structural features of the A1 protein are discussed, with special reference to its role in stabilization of the myelin membrane, and its relation to multiple sclerosis.