Inducible gene expression in eukaryotes is mainly controlled by the activity of transcriptional activator proteins, such as NF-kappa B (refs 1-3), a factor activated upon treatment of cells with phorbol esters, lipopolysaccharide, interleukin-1 and tumour necrosis factor-alpha. Activation of NF-kappa B involves release of the inhibitory subunit I kappa B from a cytoplasmic complex with the DNA-binding subunits Rel-A (formerly p65) and p50 (refs 6, 7). Cell-free experiments have suggested that protein kinase C and other kinases transfer phosphoryl groups onto I kappa B causing release of I kappa B and subsequent activation of NF-kappa B. Here we report that I kappa B-alpha (formerly MAD-3) is degraded in cells after stimulation with phorbol ester, interleukin-1, lipopolysaccharide and tumour necrosis factor-alpha, an event coincident with the appearance of active NF-kappa B. Treatment of cells with various protease inhibitors or an antioxidant completely prevented the inducible decay of I kappa B-alpha as well as the activation of NF-kappa B. Our findings suggest that the activation of NF-kappa B relies on an inducible degradation of I kappa B-alpha through a cytoplasmic, chymotrypsin-like protease. In intact cells, phosphorylation of I kappa B-alpha is apparently not sufficient for activation of NF-kappa B.