Peptide R23, consisting of residues 1091-1115 of bovine interphotoreceptor retinoid-binding protein (IRBP), had some unusual immunologic properties in Lewis rats. The peptide induced experimental autoimmune uveoretinitis and pinealitis in these rats, but only at high doses. The minimal immunopathogenic dose was found to be 100 nmol/rat. On the other hand, R23 was highly immunogenic in Lewis rats, producing cellular immunity, as measured by the lymphocyte proliferation assay, with a minimal dose of 1 nmol/rat. This unusual dissociation between the uveitogenic and immunogenic activities of R23 was attributed to different sites on the peptide, stimulating either the lymphocytes which induce disease or those which vigorously proliferate in culture. The potent immunogenicity of R23 was consistent with the peptide being immunodominant, as demonstrated by its capacity to be recognized by lymphocytes sensitized against whole IRBP and to stimulate these cells in culture to proliferate and acquire uveitogenic capacity. Likewise, lymphocytes sensitized against R23 are stimulated in culture by whole IRBP. Unexpectedly, peptide R23 was inferior to whole IRBP in its capacity to stimulate uveitogenicity in R23-sensitized lymphocytes. This finding also was attributed to the preferential stimulation by R23 of the lymphocytes specific for the putative "nonuveitogenic" site on the peptide. Peptide R23 also differs from the other tested bovine IRBP-derived peptides in the specificity of its antibodies. Unlike antibodies to the other peptides, those to R23 showed a strong cross reactivity toward whole IRBP.