Myelin basic proteins and peptides derived from them by limited cleavage with pepsin were tested for their ability to induce experimental allergic encephalomyelitis (EAE) in Lewis rats. The encephalitogenicity of the weakly active bovine protein was found to be associated with both halves of the molecule, peptides (1–88) and (89–169). Of the four smaller derivatives of peptide (1–88), peptides (1–36), (43–88), (1–42), and (37–88), only the last two were active. This demonstrated that the overlap region consisting of residues 37–42 (sequence Asp-Ser-Leu-Gly-Arg-Phe) constitutes an encephalitogenic determinant. Of the two smaller derivatives of peptide (89–169), peptides (111–169) and (89–152), only the last was active. This indicated that the second encephalitogenic determinant begins between residues 88 and 111 and ends before residue 153. This region contains the sequence Leu-Ser-Leu-Ser-Arg-Phe (residues 108–113), which is strikingly similar to that of the first encephalitogenic determinant. Studies involving the extremely encephalitogenic guinea pig protein demonstrated that virtually all of the activity was recovered in the peptides corresponding to bovine peptides (37–88) and (43–88). These peptides, but not those comprising the remainder of the protein, were active in inhibiting the passive transfer of EAE with lymph node cells from donors immunized with guinea pig spinal cord.