An ELISA for measuring serum antibody against avian encephalomyelitis virus (AEV) was evaluated for its application to the diagnosis and control of avian encephalomyelitis (AE). A scoring system was developed for this ELISA (AE ELISA-Index) so that the overall level of antibody in the flock could be presented in a single, convenient number. During suspected outbreaks of disease thought to have been caused by AEV infection, the AE ELISA-Index increased in sequential serum samples. High levels of antibody against AEV were measured in 13 flocks experiencing egg productivity problems. Variable levels of antibody activity against infectious bronchitis virus (IBV) were also observed in 11 of these flocks. The AE ELISA-Index was correlated with the embryo susceptibility test. Application of the AE ELISA has indicated that natural exposure to the virus does not occur in all flocks, and vaccination failures were detected sufficiently early for revaccination to be administered before the onset of lay.