The avian reticuloendotheliosis virus T contains within its genome the oncogene rel. The expression of this gene is responsible for the induction of lymphoid tumors in birds. Recently, the rel gene was shown to be related to the p50 DNA binding subunit of the transcription factor complex NF-kappa B. Binding sites for the NF-kappa B complex are found in the enhancer regions of a number of genes, including the immunoglobulin kappa gene and the human immunodeficiency virus long terminal repeat. In this communication we identify an activity from avian reticuloendotheliosis virus T-transformed avian lymphoid cells that binds in an electrophoretic-mobility-shift assay to an NF-kappa B binding site from the kappa enhancer. This activity contains proteins immunologically related to rel, as detected by polyclonal and monoclonal antibodies directed against v-rel. In a DNA affinity precipitation assay using the NF-kappa B site from the human immunodeficiency virus long terminal repeat, v-rel and several other proteins were identified. These data suggest that oncogenic transformation by v-rel is the result of an altered pattern of gene expression.