Nuclear factor-kappa B (NF-kappa B) is an essential. transcription factor in the control of expression of genes involved in coil growth, differentiation, inflammation, and neoplastic transformation. Previously, we reported that okadaic acid (OA), which is a specific inhibitor of serine/threonine protein phosphatases, induced apoptosis in cells of human osteosarcoma cell line MG63. However, to date, it is hot clear whether the phosphorylation status of NF-kappa B could be affected by the treatment with OA. in this report, we demonstrate that treatment of MG63 cells with OA enhanced the phosphorylation level of NF-kappa B, as judged from the results of Western blot analysis and a protein phosphatase dephosphorylation assay. The phosphorylation level of NF-kappa B was enhanced in both time- and dose-dependent manners. In the cells treated with 100nM OA for 3 h, consequential translocation of NF-kappa B from the cytosol to the nucleus occurred. Western blotting experiments with an anti-phospho-p65NF-kappa B antibody disclosed that the NF-kappa B was phosphorylated on serine 536. Furthermore, OA stimulated the transcriptional activity of NF-kappa B in MG63 cells, as judged from the results of a luciferase assay. Our findings indicate that OA elicit phosphorylation of NF-kappa B on serine 536 in MG63 cells, resulting in the translocation of phospho-NF-kappa B to the nucleus, thereby promoting transcriptional activity of genes.