Complement factor B, a serine protease playing a pivotal role in alternative pathway activation, is an acute-phase plasma protein. Previous studies have revealed that interleukin-1 (IL-1) mediates, at least in part, the acute-phase induction of factor B expression and that the IL-1-responsive element resides in the region between -553 and -478 relative to the transcription initiation site of the mouse factor B gene. In this paper, we demonstrate a specific binding site for a nuclear factor of human hepatoma HepG2 cells in this region of the factor B gene, using gel shift and methylation interference analysis. The nucleotide sequence of the binding site is closely similar to the NF kappa B or H2TF1 binding motif. The binding activity of HepG2 showed very similar specificity to that of NF kappa B or H2TF1, as shown by a competition binding assay, and was induced by IL-1 alpha treatment. A synthetic oligonucleotide corresponding to this binding site, as well as a similar sequence found in another class III complement C4 gene, conferred IL-1 responsiveness on the minimal factor B promoter. In contrast, a mutated oligonucleotide that could not bind to the HepG2 nuclear factor did not confer IL-1 responsiveness. These results suggest that IL-1 induces factor B expression via NF kappa B or a closely related factor in hepatocyte nuclei.