The activity of transcription factor NFkappaB is regulated by its subcellular localization. In most cell types, NFkappaB is sequestered in the cytoplasm due to binding of the inhibitory protein IkappaB alpha. Stimulation of cells with a wide variety of agents results in degradation of IkappaB alpha which allows translocation of NFkappaB to the nucleus. Degradation of IkappaB alpha is triggered by phosphorylation of two serine residues, i.e. Ser32 and Ser36, by as yet unknown kinases. Here we report that the mitogen-activated 90 kDa ribosomal S6 kinase (p90rsk1) is an IkappaB alpha kinase. p90rsk1 phosphorylates IkappaB alpha at Ser32 and it physically associates with IkappaB alpha in vivo. Moreover, when the function of p90rsk1 is impaired by expression of a dominant-negative mutant, IkappaB alpha degradation in response to mitogenic stimuli, e.g. 12-O-tetradecanoylphorbol 13-acetate (TPA), is inhibited. Finally, NFkappaB cannot be activated by TPA in cell lines that have low levels of p90rsk1. We conclude that p90rsk1 is an essential kinase required for phosphorylation and subsequent degradation of IkappaB alpha in response to mitogens.