Radioimmunoelectrophoresis was used to detect antibodies to myelin basic protein (BP) in sera from guinea pigs given a single encephalitogenic challenge containing homologous BP or CNS. Antibodies which were found in only 13% of sensitized guinea pigs were detected late in the course of disease and were found only in the 7S-γ2 class of immunoglobulins. With such a low incidence, it is unlikely that this antibody has any pathogenetic significance. Although the incidence of antibodies is higher in guinea pigs with mild clinical and histologic evidence of allergic encephalomyelitis (EAE) than in the entire group, the increased incidence is not sufficient to suggest a protective function for the antibody.