The purpose of the study was to map the dominant T cell epitope of the CB11 sequence of CII in RT1u haplotype rats and to determine if, when used as a synthetic peptide, it would induce tolerance to protect against CIA. A dominant epitope corresponding to residues 184-198 included in the sequence of the CB11 fragment of bovine CII was identified in proliferation assay using peptides in an epitope scanning system using synthetic peptides of 15 amino acids, overlapping by 12 amino acids. This epitope is bovine-specific, but cross-reacts with the corresponding rat peptide. Minor epitopes in the bovine CB11 sequence were also autoantigenic. Use of independently synthesized and purified 184-198 peptide confirmed its dominance in the T cell responses of arthritic rats. The peptide itself was not arthritogenic. Cells from lymph nodes draining arthritic feet were particularly responsive to the dominant peptide sequence, and showed evidence of epitope spreading to include reactions to at least four subdominant epitopes. Mucosal tolerance was successfully induced by instilling CII into the nose of rats before induction of CIA; this was found to delay the onset of disease, reduce mean disease severity, shift the anti-CII antibody response to favour antibodies of the IgGl, rather than the IgG2b isiotype, and to reduce T cell reactivity to both CII and to the 184-198 peptide. The dominant 184-198 peptide itself had the same tolerogenic effects when given nasally to rats daily, on the 4 days immediately preceding the induction of CIA. Two forms of CIA with acute and delayed disease onset were each modified by pre-treatment with the peptide. This study demonstrates that mucosal tolerance to CII can be induced by delivering it nasally in a way similar to that achieved previously by oral delivery, and that the use of an immunodominant epitope contained in a synthetic peptide will also suppress the immunologic and arthritic responses to collagen.