Activation of the inducible transcription factor NF‐kappa B involves removal of the inhibitory subunit I kappa B‐alpha from a latent cytoplasmic complex. It has been reported that I kappa B‐alpha is subject to both phosphorylation and proteolysis in the process of NF‐kappa B activation. In this study, we present evidence that the multicatalytic cytosolic protease (proteasome) is involved in the degradation of I kappa B‐alpha. Micromolar amounts of the peptide Cbz‐Ile‐Glu(O‐t‐Bu)‐Ala‐leucinal (PSI), a specific inhibitor of the chymotrypsin‐like activity of the proteasome, prevented activation of NF‐kappa B in response to tumor necrosis factor‐alpha (TNF) and okadaic acid (OA) through inhibition of I kappa B‐alpha degradation. The m‐calpain inhibitor Cbz‐Leu‐leucinal was ineffective. In the presence of PSI, a newly phosphorylated form of I kappa B‐alpha accumulated in TNF‐ and OA‐stimulated cells. However, the covalent modification of I kappa B‐alpha was not sufficient for activation of NF‐kappa B: no substantial NF‐kappa B DNA binding activity appeared in cells because the newly phosphorylated form of I kappa B‐alpha was still tightly bound to p65 NF‐kappa B. Pyrrolidinedithiocarbamate, an antioxidant inhibitor of NF‐kappa B activation which did not interfere with proteasome activities, prevented de novo phosphorylation of I kappa B‐alpha as well as its subsequent degradation. This suggests that phosphorylation of I kappa B‐alpha is equally necessary for the activation of NF‐kappa B.(ABSTRACT TRUNCATED AT 250 WORDS)