Experimental autoimmune uveoretinitis (EAU), an intraocular inflammatory disease, is induced in experimental animals by immunization with a retinal specific antigen, S-antigen (S-Ag), emulsified in complete Freund's adjuvant (CFA). The induction of EAU is enhanced by treating S-Ag-immunized animals with Bordetella pertussis. This study examined the effects of a purified component of B. pertussis, pertussis toxin (Ptx), on EAU induction as well as the mode of action of this toxin. Treatment of Lewis rats with Ptx concurrent with S-Ag and CFA enhanced EAU induction as shown by an earlier onset of disease, increased severity of ocular changes, and the reduction of the threshold amount of S-Ag needed for EAU induction. Treatment with Ptx selectively enhanced delayed-type hypersensitivity responses to S-Ag but did not affect specific antibody production. The mode of action of Ptx was analyzed by using the adoptive transfer of EAU by sensitized lymphocytes. Ptx treatment of donor rats enhanced the capacity of lymphocytes to transfer EAU. However, Ptx treatment of recipient rats on the day of cell transfer resulted in a delay in the onset of disease. These results indicate that Ptx enhances the immunopathogenic processes of EAU by enhancing lymphocyte activation and/or increasing their pathogenic activities.