To elucidate iron-regulated interferon-γ (IFN-γ) effector functions, we investigated three IFN-γ-inducible genes [intercellular adhesion molecule-1 (ICAM-1), human leukocyte antigen (HLA)-DR, guanosine 5′-triphosphate-cyclohydrolase I (GTP-CH)] in primary human monocytes and the cell line THP-1. IFN-γ increased the surface expression of ICAM-1 and HLA-DR and stimulated GTP-CH activity. Addition of iron before cytokine stimulation resulted in a dose-dependent reduction of these pathways, and iron restriction by desferrioxamine (DFO) enhanced ICAM-1, HLA-DR, and GTP-CH expression. Iron neither affected IFN-γ binding to its receptor nor IFN-γ receptor surface expression. IFN-γ-inducible mRNA expression of ICAM-1, HLA-DR, and GTP-CH was reduced by iron and increased by DFO by a transcriptional mechanism. Moreover, ICAM-1 and to a lesser extent, GTP-CH and HLA-DR mRNA expression were regulated post-transcriptionally, as iron pretreatment resulted in shortening the mRNA half-life compared with cells treated with IFN-γ alone. Thus, iron perturbations regulate IFN-γ effector pathways by transcriptional and post-transcriptional mechanisms, indicating that iron rather interferes with IFN-γ signal-transduction processes.