The I kappa B kinase complex (IKK) mediates activation of the transcription factor nuclear factor-kappa B (NF-kappa B). We previously showed that green tea polyphenols inhibited endotoxin-mediated tumor necrosis factor-alpha (TNF alpha) production by blocking NF-kappa B activation. In this study, we evaluated whether green tea polyphenols inhibit NF-kappa B by blocking IKK activity. We assessed IKK activity by detecting changes in phosphorylation of an I kappa B alpha-glutathione S-transferase (GST) fusion protein. IEC-6 cells pretreated with an extract of green tea polyphenols (GrTPs; 0--0.4 mg/ml) had diminished TNF alpha-induced IKK and NF-kappa B activity. Of the various GrTPs, (-)-epigallocatechin-3-gallate (EGCG) was the most potent inhibitor. We next examined whether EGCG inhibited activated IKK. In cytosolic extracts of TNF alpha-stimulated cells, EGCG inhibited phosphorylation of I kappa B alpha-GST (IC(50) > 18 microM) consistent with inhibition of IKK activity. Using other polyphenols, we showed that the gallate group was essential for inhibition, and antioxidants were ineffective in blocking activated IKK. Importantly, EGCG decreased IKK activity in cytosolic extracts of NIK transiently transfected cells. This latter finding showed that our findings were not related to nonspecific kinase activity. In conclusion, EGCG is an effective inhibitor of IKK activity. This may explain, at least in part, some of the reported anti-inflammatory and anticancer effects of green tea.