A monoclonal antibody (MAb), designated VR9-1, prepared against avian encephalomyelitis virus (AEV) reacted with the embryo-adapted strain, Van Roekel, two vaccine and two field strains in indirect immunofluorescent and immunoperoxidase staining of frozen sections of chicken brain, chicken embryo fibroblast and brain cells inoculated with each virus. The MAb was also used as a detector in avidin-biotin-peroxidase staining in paraffin sections where results indicate that it recognized a common epitope among strains of AEV and may be useful for detection and quantitative studies.