A series of peptides, produced from peptide 68-88 by selective enzyme cleavage, were used to define the amino acid sequences required for the induction of experimental allergic encephalomyelitis (EAE), an in vitro lymphocyte proliferative response (LPR), and serum antibody by peptide 68-88 in the Lewis rat. Here we present data that indicate that the T cell determinants for induction of EAE, an LPR, and helper function in the production of antibody are located in the same region of the molecule and that a minimum of 13 amino acids are involved; i.e., residues 71-85.