Synthetic peptide SP6 (RTPPPSG), comprising amino acid residues 98-103-Gly of myelin basic protein (MBP), and a series of peptide analogs were used to probe the structural requirements for antigenicity of a highly conserved region of a self protein. By means of a liquid-phase radioimmunoassay, antibody responses directed toward this determinant in both multi-specific anti-MBP and monospecific anti-peptide antisera were measured. The specificities of the antibodies present in the anti-MBP and anti-peptide antisera were examined by an equilibrium competitive inhibition radioimmunoassay by using the set of related peptides, as well as intact MBP from different species. Although the fine specificities of the reagent antisera differed, competitive inhibition analyses with intact MBP revealed a cross-reactive determinant involving residues 99-100 (Thr-Pro). This suggests that the neighborhood of the triprolyl region of MBP, despite its strong phylogenetic conservation, serves as an immunogen for humoral responses whether presented as a hapten-carrier conjugate or in the context of intact MBP. The latter supports the contention that the general antigenicity of a protein need not require sequence differences between the immunizing protein and its counterpart in the host.