The effect of pertussis toxin (PT) on experimental autoimmune uveoretinitis (EAU) induced by immunization with S-antigen was examined in rats. Intravenous administration of PT (2 micrograms/rat) initiated the development of EAU in rats that had been made resistant to the induction of EAU by immunization with S-antigen and complete Freund's adjuvant (CFA). The capacity of PT to promote EAU was also demonstrated by a marked augmentation of the inflammation in EAU eyes of rats susceptible to the induction of EAU. PT was most effective when it was given from the day before to the day after immunization with S-antigen. However the induction of EAU was promoted by the injection of PT even 7 days before and 14 days after immunization. The clinical and histopathological findings of the EAU produced by the additional PT treatment were described and the mechanisms by which PT augmented the induction of EAU were discussed.