Interleukin-8 (IL-8) is a key component of the innate immune response because of its ability to recruit inflammatory cells to sites of inflammation. Although the effects of IL-8 are largely beneficial, aberrant expression of IL-8 is known to contribute to a number of pathologic states. Interferon-β (IFN-β), an antiviral cytokine, is known to inhibit the expression of IL-8, although the exact mechanism by which this occurs has yet to be elucidated. In this study, we dissect the role of each member of the IFN-stimulated gene factor 3 (ISGF3) signaling complex in contributing to IFN-β inhibition of IL-8 gene expression. To date, no IFN-stimulated response element (ISRE) (the DNA binding target for ISGF3) has been identified within the promoter region of the IL-8 gene. We conclude, through use of cell lines deficient for ISGF3 components, that all three members of this complex, Stat1, Stat2, and IFN regulatory factor-9 (IRF-9), are required for IFN-β-mediated inhibition of IL-8 expression. In contrast to positive signaling by ISGF3 to activate gene expression, we find that the transactivation domains of Stat1 and Stat2 are not essential to IFN-β inhibition of IL-8. Taken together, these data define the role of the ISGF3 members in IFN-β inhibitory signaling. © 2008 Mary Ann Liebert, Inc.