A general paradigm in signal transduction is ligand-induced feedback inhibition and the desensitization of signaling. We found that subthreshold concentrations of interferon-γ (IFN-γ), which did not activate macrophages, increased their sensitivity to subsequent IFN-γ stimulation; this resulted in increased signal transducer and activator of transcription 1 (STAT1) activation and increased IFN-γ–dependent gene activation. Sensitization of IFN-γ signaling was mediated by the induction of STAT1 expression by low doses of IFN-γ that did not effectively induce feedback inhibition. IFN-γ signaling was sensitized in vivo after IFN-γ injection, and STAT1 expression was increased after injection of lipopolysaccharide and in rheumatoid arthritis synovial cells. These results identify a mechanism that sensitizes macrophages to low concentrations of IFN-γ and regulates IFN-γ responses in acute and chronic inflammation.