Activation of a large multisubunit protein kinase, called the inhibitor kappaB kinase (IKK) complex, is central to the induction of the family of transcription factors nuclear factor kappaB. IKK is comprised of two catalytic subunits, IKKalpha and IKKbeta, and a regulatory IKKgamma subunit. It is known that the catalytic IKKbeta and regulatory IKKgamma subunits associate through interactions mediated by the N-terminal region of IKKgamma and an 11-mer peptide located near the C-terminus of IKKbeta. In this study, we have defined the minimal IKKgamma segment that binds IKKbeta and determined the binding affinity of the IKKbeta/IKKgamma complex. We identified that the N-terminal segment spanning residues 40-130 of IKKgamma binds the IKKbeta C-terminal domain (residues 665-756) with Kd approximately 25 nM. Several smaller N-terminal IKKgamma deletion mutants within the N-terminal 130 residues, although in some cases retained IKKbeta binding activity, showed a tendency to aggregate and formed covalently linked complexes. However, expansion of the C-terminus of these fragments to residue 210 completely changed the solution behavior of the IKKgamma N-terminus without affecting the IKKbeta binding affinity. We also found that the IKKbeta C-terminal domain formed a dimer in solution and the basic unit of the IKKbeta/IKKgamma complex was a dimer/dimer.