IkappaB [inhibitor of nuclear factor kappaB (NF-kappaB)] kinase (IKK) phosphorylates IkappaB inhibitory proteins, causing their degradation and activation of transcription factor NF-kappaB, a master activator of inflammatory responses. IKK is composed of three subunits-IKKalpha and IKKbeta, which are highly similar protein kinases, and IKKgamma, a regulatory subunit. In mammalian cells, phosphorylation of two sites at the activation loop of IKKbeta was essential for activation of IKK by tumor necrosis factor and interleukin-1. Elimination of equivalent sites in IKKalpha, however, did not interfere with IKK activation. Thus, IKKbeta, not IKKalpha, is the target for proinflammatory stimuli. Once activated, IKKbeta autophosphorylated at a carboxyl-terminal serine cluster. Such phosphorylation decreased IKK activity and may prevent prolonged activation of the inflammatory response.