The pathogenicity and transmission of a field isolate of reticuloendotheliosis virus (REV) was studied using an experimental model in Japanese quail. Oncogenicity was also evaluated after inoculations in chickens and turkeys. The original REV (designated APC-566) was isolated from Attwater's prairie chickens (Tympanuchus cupido attwateri), an endangered wild avian species of the southern United States. The transmissibility of the REV isolate was studied in young naive Japanese quail in contact with experimentally infected quail. Vertical transmission was not detected by virus isolation and indirect immunofluorescence. Seroconversion was detected in few contact quails, suggesting horizontal transmission. The APC-566 isolate induced tumors beginning at 6 wk of age in quails infected as embryos. Most of the tumors detected in Japanese quail were lymphosarcomas, and 81% of these neoplasias contained CD3 cells by immunoperoxidase. REV APC-566 was also oncogenic in chickens and turkeys infected at 1 day of age, with tumors appearing as early as 58 days after infection in chickens and at 13 wk of age in turkeys. This study was conducted in part as an attempt to understand the potential for pathogenicity and transmission of REV isolated from endangered avian species.