Nuclear factor kappa B (NF-kappa B) is stored in the cytoplasm as an inactive form through interaction with I kappa B. Stimulation of cells leads to a rapid phosphorylation of I kappa B alpha, which is presumed to be important for the subsequent degradation. We have recently reported the establishment of a lipopolysaccharide (LPS)-dependent cell-free activation system of NF-kappa B in association with the induction of I kappa B alpha phosphorylation. In this study, we have identified a kinase in cell extracts from the LPS-stimulated human monocytic cell line, THP-1, that specifically binds and phosphorylates I kappa B alpha. LPS stimulation transiently enhanced the I kappa B alpha-bound kinase activity in THP-1 cells. Mutational analyses of I kappa B alpha and competition experiments with the synthetic peptides identified major phosphorylation sites by the bound kinase as Ser and Thr residues in the C-terminal acidic domain of I kappa B alpha. Moreover, we show that the peptide, corresponding to the C-terminal acidic domain of I kappa B alpha, blocked the LPS-induced NF-kappa B activation as well as inducible phosphorylation of endogenous I kappa B alpha in a cell-free system using THP-1 cells. These results suggested that the bound kinase is involved in the signaling pathway of LPS by inducing the phosphorylation of the C-terminal region of I kappa B alpha and subsequent dissociation of the NF-kappa B.I kappa B alpha complex.