In a cotransfection assay, the human T-cell leukemia virus type I Tax1 gene product specifically activated transcription from the mouse tumor necrosis factor alpha-promoter. The activation patterns of 5' deletion mutants, artificial enhancer constructs, and point mutations in the promoter indicate that the major Tax1-responsive element is a site at position -655 which binds the NF-kappa-B/rel and NF-GMa transcription factors.