NUCLEAR factor κB" (NF-κB"), which was first detected by its binding to the κB" site in the immunoglobulin κ-gene enhancer¹, is important for the regulated expression of the κ-gene^2,3 and is partly responsible for the induction in appropriate cells of inter-leukin-2 (IL-2) ⁴, IL-2α receptor⁵, β-interferon^6,7 and serum amyloid A protein⁸. NF-κ" is present as a nuclear DNA-binding protein in B lymphocytes and mature macrophages, but is found in the cytoplasm of many cells in a form unable to bind to DNA⁹. The cytoplasmic form is bound to an inhibitor protein, IκB, from which it can be released in vitro by deoxycholate and other agents¹⁰. Activation of cells by various agents, notably the phorbol esters that stimulate protein kinase C (PKC), leads to dissociation in vivo of the NF-κB/IκB complex and migration of NF-κB to the nucleus⁹. Therefore, it acts as a second messenger system, transducing activation signals from the cytoplasm to the nucleus. To elucidate the mechanism of signal transfer, we have used an in vitro system in which addition ofpurified protein kinases to a partially purified NF-κB/IκB complex leads to the activation of the DNA-binding activity of NF-κB. Using gel retardation assays we found that PKC, cyclic AMP-dependent protein kinase (PKA) and a haem-regulated eIF-2 kinase (HRI) could activate NF-κB in vitro, whereas casein kinase II was ineffective. To determine the target for the protein kinases we purified and characterized both NF-κB and IκB and found that IκB is phosphorylated and inactivated in the presence of PKC and HRI but not PKA.