We studied synthetic peptides that correspond to two regions of the guinea pig myelin basic protein (MBP) molecule which elicit experimental autoimmune encephalomyelitis (EAE) in DA rats. Using truncated peptides, we determined that two encephalitogenic epitopes reside within MBP63-81, a major determinant defined by MBP residues, 63-76, and a minor encephalitogenic epitope defined by residues, 66-81. Experiments with alanine-substituted analogs of MBP63-76 revealed that the HYGSLP sequence is critical for encephalitogenicity. The core epitope within a second encephalitogenic region, MBP101-120, was defined by residues, 106-119. Studies with analogs of this sequence indicated that residues, Leu 111, Phe 114 and Trp 116 are important for T-cell responsiveness.