The facts that fibronectin (FN) mRNA is elevated in cells expressing human T cell leukemia virus type I (HTLV-I) Tax protein and that Tax is known to transactivate the cellular cAMP-response element (CRE) prompted us to examine whether Tax activates the FN promoter of which CRE is thought to play an important role. We showed that Tax transactivated the FN promoter in Jurkat cells. Deletion analyses showed that the response-element resides within the promoter region of -69 bp and that an NF-kappa B-binding site at -41 bp is involved in the Tax-activation of the FN promoter. GeI-shift assays showed that DNA-protein complexes binding to the NF-kappa B site, composed of NF-kappa B p50/p65, were induced on the NF-kappa B motif at -41 bp by Tax. Overexpression of NF-kappa B enhanced the Tax-activation of the FN promoter. Our study shows that the FN promoter is transactivated by Tax through the NF-kappa B pathway,