The N-terminal peptide (Ac1-9) of myelin basic protein (MBP) is the immunodominant encephalitogenic epitope in H-2Umice. Previous studies have defined the role of amino acid residue 4 in binding to I-Au. Accordingly, substitutions at this residue have generated peptides spanning a wide range of affinities for the MHC. In the present study, we have tested the toterogenlcity of three of these peptides, Ad-9, Ac1-9[4A] and Ac1-9[4Y], by administering these to mice i.p. in the absence of adjuvant Significantly, mice treated with the high affinity analogues Ac1-9[4A] and Ac1-9[4Y]prior to immunization became less susceptible to Ad1-9-induced experimental autoimmune encephalomyelitis (EAE), whereas those given the low affinity peptide Ac1-9 were only moderately protected. T cell priming, as assessed byin vitroproliferate and lymphokine assays, demonstrated a direct correlation between the level of disease inhibition and T cell unresponsiveness. In treatment studies, Ac1-9 and Ac1-9[4Y] were also shown to be effective when given on the first day of disease onset. Priming of T cells, when measured by proliferationin vitro, however, became more resistant to inactivation when soluble peptides were administered close to the day of assay. Kinetic studies revealed that tolerance could be achieved in primed mice but that this takes time to develop. Two conclusions can be drawn from this study: (I) administration of peptide in solution is effective in prevention of EAE both before and after autoreactive T cell activation, and (II) high affinity analogues of self-peptides inactivate their cognate T cells readily whereas lower affinity peptides, being less efficient in tolerance induction, may allow potentially autoreactive T cells to persist in healthy individuals.