Intravenous immune globulin (IVIG) suppresses autoantibody-mediated inflammation by inducing and activating the inhibitory Fc receptor FcgammaRIIb and downstream negative signaling pathways. We investigated the effects of IVIG on cellular responses to interferon-gamma (IFN-gamma), a potent macrophage activator that exacerbates inflammation. Our study showed that IVIG blocked IFN-gamma signaling and IFN-gamma-induced gene expression and suppressed IFN-gamma function in vivo during immune responses to Listeria monocytogenes and in an IFN-gamma-enhanced model of immune thrombocytopenic purpura. The mechanism of inhibition of IFN-gamma signaling was suppression of expression of the IFNGR2 subunit of the IFN-gamma receptor. The inhibitory effect of IVIG was mediated at least in part by soluble immune complexes and was dependent on FcgammaRIII but independent of FcgammaRIIb. These results reveal an unexpected inhibitory role for the activating FcgammaRIII in mediating suppression of IFN-gamma signaling and suggest that inhibition of macrophage responses to IFN-gamma contributes to the anti-inflammatory properties of IVIG.