Encephalitogenic, immunogenic properties of the polypeptide fraction of myelin basic protein (FBP) and CNS lesions have been examined in animals with experimental allergic encephalomyelitis (EAE). FBP was isolated from bovine spinal cord using column chromatography. Administration of 1.0 or 0.1 microgram FBP mixed with complete Freund adjuvant caused neurological and histological EAE manifestations in 76% and 26% of guinea-pigs, respectively. Circulating anti-FBP antibodies were not found in sensitized animals, whereas the incidence and intensity of skin reaction of delayed type hypersensitivity to FBP correlated with the development of EAE and the onset of the disease. Perivascular cell infiltration and demyelination noted in the spinal cord and brain of guinea-pigs were similar to those observed after inoculation of the brain white matter or brain tissue homogenate.