A large protein complex mediates the phosphorylation of the inhibitor of kappa B (I kappa B), which results in the activation of nuclear factor kappa B (NF-kappa B). Two subunits of this complex, I kappa B kinase alpha (IKK alpha) and I kappa B kinase beta (IKK beta), are required for NF-kappa B activation. Purified recombinant IKK alpha and IKK beta expressed in insect cells were used to demonstrate that each protein can directly phosphorylate I kappa B proteins. IKK alpha and IKK beta were found to form both homodimers and heterodimers. Both IKK alpha and IKK beta phosphorylated I kappa B bound to NF-kappa B more efficiently than they phosphorylated free I kappa B. This result explains how free I kappa B can accumulate in cells in which IKK is still active and thus can contribute to the termination of NF-kappa B activation.