Summary: A recently described neurotropic virus, the West Nile virus, was found to be immunologically related to, but distinct from the viruses of St. Louis and Japanese B encephalitis. Antiserum against the West Nile virus does not neutralize either of these viruses. Vaccination with either of the three viruses causes enhanced resistance only to the homologous strain. The immunological relation of the viruses is shown by the fact that antisera against the St. Louis virus may cross-neutralize the West Nile virus and antisera against Japanese B virus have some power to cross-neutralize both the West Nile and the St. Louis viruses.