A direct binding assay specific for IA^s molecules has been developed and Its immunological relevance validated by examining, for a panel of nine different synthetic peptides, the correlation between their capacity to bind purified IA^s and to inhibit IA^srestricted antigen presentation. The IA^s assay thus developed has then been used to study the IA^s binding affinity of a set of overlapping peptides spanning the entire myelin basic protein (MBP). It was found that the encephalitogenlc MBP region corresponds to peptides with high MHC binding affinities. Other regions of the MBP that have not been described as being pathogenic in the context of IA^s molecules have also been found to be high IA^s binders, suggesting that variables other than MHC affinity are also Involved in determining the pathogenic potential of self-derived determinants.