NF-κB, a key regulator of the cellular inflammatory and immune response, is activated by the HTLV-I transforming and transactivating protein Tax. We show that Tax binds to the amino terminus of the protein kinase MEKK1, a component of an IκB kinase complex, and stimulates MEKK1 kinase activity. Tax expression increases the activity of IκB kinase β (IKKβ) to enhance phosphorylation of serine residues in IκBα that lead to its degradation. Dominant negative mutants of both IKKβ and MEKK1 prevent Tax activation of the NF-κB pathway. Furthermore, recombinant MEKK1 stimulates IKKβ phosphorylation of IκBα. Thus, Tax-mediated increases in NF-κB nuclear translocation result from direct interactions of Tax and MEKK1 leading to enhanced IKKβ phosphorylation of IκBα.