An examination of antibody to reticuloendotheliosis virus (REV) in sera of chickens and other avian species collected from various districts in Japan was carried out. Agar gel immunodiffusion test (AGID) to the chicken sera revealed positive results in 25.9% of the 54 farms, 21.4% of the 126 flocks and 14.3% of the 1,892 individual chickens. All the AGID positive sera also were positive in indirect immunofluorescence and/or virus neutralization tests. In avian species other than chickens, Peking ducks (1/120; 0.4%) and pheasants (4/27; 14.8%) were AGID test positive. This is the first study conducted in Japan.