Brown Norway (BN) rats are much less susceptible to experimental allergic encephalomyelitis (EAE) than Lewis rats. Nevertheless, BN rats developed severe EAE, even paralysis, when immunized with rat spinal cord and carbonyl iron adjuvant. Complete Freund's adjuvant (CFA) was much less effective. The use of both CFA and pertussis vaccine with rat cord was moderately, but not consistently, effective. Guinea pig spinal cord was weakly encephalitogenic to BN rats with all adjuvant combinations. We were not able to produce EAE in BN rats with purified myelin basic protein from either rat or guinea pig. Inoculations directly into lymph nodes or into the blood stream proved that the low susceptibility of BN rats was not due to lack of absorption from the site of inoculation, but may be related to peculiarities of processing antigen in draining lymph nodes. The severity of EAE in F1 hybrids was intermediate between the BN and Lewis parental strains when tested with an immunizing procedure of appropriate strength. The fact that F1 hybrids were less reactive than Lewis mandates modification of the theory that susceptibility to EAE is inherited through a single autosomal dominant gene.