Eleven strains of equine encephalomyelitic virus isolated from different localities of the United States could be divided serologically into 2 groups, one from the West (California, Nevada, Utah (Nos. 1 and 5), Colorado, South Dakota and Minnesota) and one from the East (New Jersey (Nos. 1, 2 and 3), Maryland (Md. 1) and Delaware). The strain of virus obtained from the Argentine was classified with the western series, while that from Russia was immunologically distinct from the others and varied in a few characteristics. There was no in vitro cross neutralization nor in vivo cross protection between serums of any group when tested against the heterologous viruses. Protection occurred, however, between homologous serums and viruses of each type. Animals immunized to each member of the 3 divisions showed a constant tissue immunity within their own groups when tested intracerebrally, but cross injection experiments were not conclusive, except for the Russian virus which was differentiated from the others. A certain percentage of guinea pigs immune to the western American strains showed immunity when tested with the eastern and vice versa. The numbers of animals succumbing or surviving after injection with heterologous strains was approximately proportional for both eastern and western American varieties. Young guinea pigs born of immune parents and immune to their respective American viruses were more susceptible to injections of the heterologous strains than older animals hyperimmunized with massive doses of live virus. The American eastern strains of virus were found to be far more invasive and potent than the western, both by intracerebral and by intradermal inoculation of guinea pigs, although the virulence of both could be enhanced by repeated animal passage. Rabbits were found to be more susceptible to the eastern american strains than to any of the others and could even be infected intravenously. The Russian virus was also invasive for rabbits when given intracerebrally but not when into the veins. Comparison of potency was best demonstrated by titration methods. Although 3 serological groups of equine encephalomyelitic virus may be differentiated, there is a probability that the strains isolated from horses in the various localities have sprung from a common root stock and should therefore be considered as varieties or subvarieties of the identical virus causing the same clinical disease.