NF-κB family of transcription factors plays a pivotal role in regulation of immune and inflammatory responses. NF-κB is known to function by binding to the κB enhancer and directly activating target gene transcription. Here we demonstrate another function of NF-κB, in which the nfκb1 gene product p105 regulates MAP kinase signaling triggered by the bacterial component lipopolysaccharide. p105 exerts this signaling function by controlling the stability and function of an upstream kinase, Tpl2. In macrophages, Tpl2 forms a stable and inactive complex with p105, and activation of Tpl2 involves its dissociation from p105 and subsequent degradation. Thus, p105 functions as a physiological partner and inhibitor of Tpl2, which provides an example of how a transcription factor component regulates upstream signaling events.