The production of eotaxin, which is a critical mediator for airway inflammation, is inhibited by IFN-γ. Here, we investigated the precise mechanisms underlying IFN-γ-dependent inhibition of eotaxin production using mouse embryonic fibroblasts (MEF). MEF produced high levels of eotaxin in STAT6-dependent manner when they were cultured with both IL-4 and TNF-α. However, the eotaxin production by MEF was strongly inhibited by addition of IFN-γ. Western-blotting analysis demonstrated that IFN-γ downmodulated STAT6 phosphorylation induced by IL-4 and TNF-α. Moreover, IFN-γ did not exhibit its inhibitory effect on both STAT6-phosphorylation and eotaxin production in MEF obtained from deficient mice in STAT1, a key molecule of IFN-γ signaling. We also demonstrated that SOCS-1, a potent inhibitory molecule of IL-4 signaling, was induced by IFN-γ in STAT1-dependent manner. This indicated that SOCS-1 might be involved in IFN-γ-mediated STAT1-dependent inhibition of eotaxin production. In SOCS-1 - / - MEF, IFN-γ inhibited neither STAT6 phosphorylation nor eotaxin production induced by IL-4 and TNF-α. Conversely, retroviral transduction of SOCS-1 into MEF inhibited STAT6 phosphorylation and eotaxin production induced by IL-4 and TNF-α, in the absence of IFN-γ. Thus, we demonstrated that IFN-γ-induced inhibition of STAT6 phosphorylation and eotaxin production were mediated by SOCS-1 induced in STAT1-dependent manner.