Immunocytochemical studies showed that monoclonal antibodies to bovine retinal S-antigen recognize an immunoreactive protein present in the bovine ciliary body epithelium. This protein was purified by DEAE-agarose chromatography and found to have a M.W. of 28,000 daltons by SDS polyacrylamide gel electrophoresis. Crude and purified preparations of this protein cross-react with polyclonal antibodies to retinal S-antigen. The relative reactivity of S-antigen monoclonal antibodies to ciliary body protein was determined by ELISA. Lewis rats immunized with crude and purified ciliary body protein failed to develop experimental uveitis. The results indicate that the ciliary body protein contains an antigenic peptide domain which is similar to that of S-antigen but lacks the immunopathogenic domain.