As examined by the luciferase assay, a negative regulatory region including the NF kappa B motif was found in the 5'-flanking region of the 12-lipoxygenase gene in human erythroleukemia cells. The negative control was abolished by a site-specific mutation of the NF kappa B motif. Probes including the NF kappa B region gave positive bands upon a gel-shift assay. The bands were super-shifted by antibodies for NF kappa B p50, NF kappa B p65 and c-Rel, and were lost by a NF kappa B competitor DNA. Furthermore, the NF kappa B sequence was protected in DNase I footprinting. Thus, two kinds of heterodimer (p50 and p65; p50 and c-Rel) seemed to control the over-expression of the human 12-lipoxygenase gene.