Antigens capable of fixing complement specifically with the appropriate antibodies have been prepared from brain tissue of hamsters and mice infected with the viruses of St. Louis, Japanese, Western, and Eastern encephalitis, and with the West Nile virus. The antigens were freed of the material which reacts with normal serum by means of centrifugation at relatively high speed. In addition, the infectivity of the preparation was destroyed by irradiation with ultraviolet light.
Cross reactions were demonstrated by means of the complement-fixation technique with materials from animals infected with the viruses of Eastern and Western equine encephalitis. No relationship was detectable by this procedure between St. Louis, Japanese, and West Nile viruses. These findings emphasize the need for further investigation and correlation of the immunological reactions of the groups of neurotropic viruses, since the equine agents are apparently unrelated when studied by neutralization and cross-immunity tests while these methods provide evidence of the presence of common antigenic structures in the St. Louis, Japanese, and West Nile agents.