The initial phase of inflammation is accompanied by dramatic changes in the concentrations of certain plasma proteins. Interleukin-6 (IL-6) is an important inducer of these acute phase proteins at the transcriptional level. The recently cloned nuclear factor NF-IL6, a potent trans-acting regulator of IL-6 gene expression, has a region that is highly homologous to the liver-specific transcriptional factor C/EBP. Both factors recognize the same nucleotide sequence. In this study the recombinant NF-IL6 was shown to interact with the IL-6-responsive elements (IL-6REs) identified in the promoter region of several acute phase protein genes whose activity increases during the acute phase reaction. Furthermore, in competition experiments, formation of all the DNA-protein complexes by the IL-6RE and IL-6-treated hepatoma cell extracts was specifically decreased by adding either the 14-bp NF-IL6 binding motif identified in the IL-6 promoter or the antibody against the recombinant NF-IL6. NF-IL6 was expressed at a minor level in mouse liver, but was dramatically induced after stimulation with IL-6. In contrast, the amount of C/EBP mRNA decreased considerably after IL-6 stimulation. These results indicate that the NF-IL6 that regulated IL-6 expression was also involved in regulation of expression of the acute phase protein genes. The ability of NF-IL6 to replace C/EBP may explain the positive and negative acute phase responses induced by IL-6.