The addition of complete Freund's adjuvant (CFA) to encephalitogen is required for the induction of experimental allergic encephalomyelitis (EAE). On the other hand, administration of encephalitogen in incomplete Freund's adjuvant (IFA) protects the animal from the development of EAE. It is shown in this work that injection of homologous central nervous system (CNS) tissue or myelin basic protein (BP) in IFA, before challenge with CNS tissue in CFA, accelerated the onset of the disease in Hartley guinea pigs. It also appeared to protect the animals, however, because 22% of the group did not develop EAE at all, and in those which did, the disease was not as lethal as in controls. To produce this accelerated form of EAE with encephalitogen in IFA required (1) a time interval shorter than 9 days between the first injection and challenge and (2) that the first injection and the challenge be done in the same site, which could be hind or front foot pads but not the nuchal area. The results indicated that 'priming' by encephalitogen in IFA occurred when this two-step induction procedure was used. The experimental conditions may have bypassed suppressive mechanisms.