Reticuloendotheliosis virus (REV) infection can result in immunosuppression, a runting syndrome, high mortality, acute reticulum cell neoplasia, or T-cell and/or B-cell lymphomas, in a variety of domestic and wild birds. Histopathological changes in REV infection are not sufficient to differentiate it from avian lymphoid leukosis and Marek's disease, and currently there are no available in situ diagnostic methods for detection of active REV presence in pathologic specimens. To develop immunohistochemistry and in situ hybridization assays for detection of REV active infections, experimentally inoculated Japanese quail embryos, and archived formalin-fixed paraffin-embedded tissues from natural and experimental reticuloendotheliosis cases in chickens and turkeys, were examined. The in situ hybridization and immunohistochemistry assays proved to be efficient for the detection of several REV strains in Japanese quail embryos during active infection, whereas these assays were much less sensitive when applied to archived tissue samples from chronically infected birds with lymphoid tumours. The diagnostic assays developed in this study have potential as diagnostic tools for detection of active REV infections.