Blocking of the Ag presentation function of MHC molecules by competitor peptides has been proposed as a potential immunotherapy for MHC-associated autoimmune diseases. Despite the fact that successful inhibition of experimental autoimmune encephalomyelitis (EAE) by coimmunization with competitor peptides had been achieved, it remained questionable whether the in vivo activity of such peptides was solely the result of MHC blockade. In the peptide MBP72-85-induced EAE model in Lewis rats, we designed a single amino acid-substituted analogue of MBP72-85 with a superior MHC binding capacity, and with the capacity to activate encephalitogenic MBP72-85-specific T cells. Subsequently, two well-defined competitor peptides, one EAE related and one non-EAE related, were studied for their respective efficacies to inhibit the in vitro proliferation of an encephalitogenic T cell clone induced by the original MBP72-85 or the superior MHC binding analogue peptide. It appeared that the response to MBP72-85 was inhibited very efficiently by both competitor peptides, whereas the response to the superior MHC binding analogue peptide was not. Co-immunization of either the related or non-related competitor peptide together with MBP72-85 inhibited EAE induction in a concentration-dependent manner. In such protected rats, polyclonal T cell responses against MBP72-85 were dramatically decreased. However, EAE induced by the stronger MHC binding MBP72-85 analogue could not be inhibited by either of the competitor peptides. Moreover, in these rats, T cell priming for both MBP72-85 and the MBP72-85 analogue was not inhibited. These results show that competition for MHC binding in vivo could lead to inhibition of EAE induction.