We have previously identified a repressor element in the TNF-alpha promoter (-280 to -172) by deletion analysis. When this 108 bp repressor element was placed in front of a heterologous promoter containing an NF-kappa B binding site, less repression was observed. When this element was dimerized and placed in front of the positive element (-125 to -102) of the TNF-alpha promoter, instead of repression, activation was observed. There is an NF-kappa B-like site in the 108 bp repressor region (-211 to -202) and our gel retardation analysis showed that this site and a known NF-kappa B binding site both could compete for one of the specific protein complexes formed on the 108 bp probe. To test the functionality of this NF-kappa B-like site, we mutagenized the critical GGGG sequence to ATCC. Contrary to our prediction, such a mutation blocked the repressor function of the 108 bp element. This suggests that the NF-kappa B-like site is an essential sequence for the repressor function of the 108 bp repressor element.