An enzyme-linked immunosorbent assay (ELISA) for the detection of reticuloendotheliosis virus (REV) is described. The assay is based on antiserum produced in rabbits against the group-specific (gs) antigen of REV which has a molecular weight of 30 kDa (p30). The p30 for immunisation was obtained by electro-elution from polyacrylamide gels after separation of purified REV by electrophoresis. Western blotting indicated that the antiserum produced was specific for p30 of REV. The double antibody sandwich ELISA developed using the rabbit anti-p30 was found to be specific for REV and highly efficient for the detection of REV in a variety of samples when compared with the indirect immunofluorescence assay. The smallest amount of REV detectable was 2,000 fluorescence forming units. Serum, vaginal/ cloacal swab samples and egg albumens obtained from experimentally infected REV-viraemic chickens were all consistently positive for the ¿?i-antigen of REV. Some serum samples from non-viraemic chickens, however, gave 'false positive' reactions in the ELISA. Albumens gave higher readings in ELISA than vaginal/cloacal swab samples and should therefore be the sample of choice for screening flocks for REV.