Experimental allergic encephalomyelitis (EAE) is a cell-mediated autoimmune response directed toward a component of central nervous system (CNS) tissue, myelin basic protein (BP). Injection of animals with either whole CNS tissue or purified BP in complete Freund's adjuvant (CFA) induces severe and usually fatal disease. Preimmunization of animals with BP in incomplete Freund's adjuvant (IFA) prevents EAE. We have examined the relative abilities of whole guinea pig BP and its fragments to protect guinea pigs from subsequent EAE induction. The data suggest that the presence of the intact encephalitogenic site (residues 113-121) in the molecules used for preimmunization is necessary but may not be sufficient for complete protection against EAE induction.