Several members of the tumour-necrosis/nerve-growth factor (TNF/NGF) receptor family activate the transcription factor NF-kappa B through a common adaptor protein, Traf2 (refs 1-5), whereas the interleukin 1 type-I receptor activates NF-kappa B independently of Traf2 (ref. 4). We have now cloned a new protein kinase, NIK, which binds to Traf2 and stimulates NF-kappa B activity. This kinase shares sequence similarity with several MAPKK kinases. Expression in cells of kinase-deficient NIK mutants fails to stimulate NF-kappa B and blocks its induction by TNF, by either of the two TNF receptors or by the receptor CD95 (Fas/Apo-1), and by TRADD, RIP and MORT1/FADD, which are adaptor proteins that bind to these receptors. It also blocked NF-kappa B induction by interleukin-l. Our findings indicate that NIK participates in an NF-kappa B-inducing signalling cascade common to receptors of the TNF/NGF family and to the interleukin-1 type-I receptor.