This report describes expression of heritable reticuloendotheliosis virus (REV) vector ME111 in 20 independent lines of transgenic chickens. The results are strikingly different from studies of Moloney virus in transgenic mice, where restricted expression of inherited proviruses has led to their use primarily as insertional mutagens rather than general agents for gene transfer. In contrast, the REV ME111 provirus is actively transcribed in a variety of tissues from transgenic chickens, is expressed from transcriptional control elements present in the long terminal repeat of the provirus, and codes for active neomycin phosphotransferase II. The REV vector system as applied to the chicken represents a departure from the long-established paradigm of retroviral transgenes in mice and provides a new approach to the study of avian biology.