Highly purified synthetic peptides representing portions of the 68-86 sequence of guinea pig (GP) myelin basic protein (GPMBP) were used to define the N- and C-termini of encephalitogenic determinants that cause experimental autoimmune encephalomyelitis (EAE) in Lewis rats. Each peptide was tested for: (a) induction of EAE, (b)in vitro potentiation of EAE transfer activity by GPMBP-sensitized lymph node cells (LNC), (c) in vitro proliferation of GPMBP-sensitized LNC, and (d) in vitro proliferation of a GPMBP-reactive line of EAE-inducing T cells. In these bioassays, the general rank order of potency was: GPMBP greater than or equal to GP68-86 greater than or equal to GP72-86 greater than [G84]GP68-86 greater than or equal to GP68-84 much greater than GP75-85 greater than or equal to GP75-84 = virtually no activity. These results demonstrate that the encephalitogenic region is bounded by the 72-74 and 84-86 sequences. Further evidence presented herein indicates that the 75-84 sequence contains the primary antigenic features required for specific T cell recognition of the encephalitogenic region.