The virus of pneumoencephalitis was found to be immunologically identical with the virus of Newcastle disease. Antisera for the viruses of Newcastle disease and fowl plague, diluted or undiluted with saline, were each mixed with embryocultured pneumoencephalitis virus. The mixts. were injd. intramusc. in doses of 0.1 ml. into 61-day-old chickens. The chickens were not infected by the virus when it was mixed with Newcastle disease immune serum. The virus was not affected, however, by mixing it with fowl plague immune serum.