The IE2p86 protein of human cytomegalovirus is an essential activator of early- and late-phase viral gene expression. Whilst IE2p86 activates expression of a number of cellular genes, it also represses certain cellular genes, particularly those activated by nuclear factor kappaB (NF-kappaB). As the interleukin-6 (IL-6) promoter can be activated by both NF-kappaB and IE2p86, it was examined whether there is competition between these two factors. Here, it is reported that both wild-type and mutant IE2p86 can block activation of the IL-6 promoter in response to interleukin-1beta. By using an artificial activator in which the activation domain of NF-kappaB is directed to the promoter by the GAL4 DNA-binding domain, it is shown that the mutant form of IE2p86 can inhibit NF-kappaB-mediated activation at a step subsequent to promoter recruitment. These data therefore suggest a novel mechanism for inhibition of NF-kappaB by IE2p86.