The human MxA protein is one of the interferon-inducible proteins that inhibits multiplication of influenza virus and other viruses. To clarify the control mechanism of its expression, we prepared a series of mutant MxA promoters and identified a 30 nucleotides long cis-acting interferon-responsive element by transient transfection assay. Its nucleotide sequence is somewhat similar to that of ISRE (interferon-stimulated response element), suggesting that the regulation of MxA mRNA synthesis is under the control of some ISRE binding factor such as ISGF-3 (interferon-stimulated gene factor-3).