Interferons are a family of proteins first identified by their ability to induce cellular resistance to infection by many viruses. In addition to the antiviral properties it shares with the α- and β-interferons, γ-interferon (IFN-γ), a lymphokine secreted by activated T cells, activates macrophages, stimulates B cells, increases fibroblast and endothelial cell resistance to many non-viral intracellular parasites and modulates cell-surface proteins central to immune cell regulation^1–13. To identify molecules involved in the IFN-γ response and characterize their modulation, we have isolated genes that are induced following recombinant IFN-γ treatment of U937 cells, a histiocytic lymphoma cell line with monocytic characteristics^14,15. We report here the molecular cloning and characterization of a gene regulated by rIFN-γ in U937 cells as well as in human mononuclear cells, fibroblasts and endothelial cells. Messenger RNA from this gene is induced within 30 min of rIFN-γ treatment and demonstrates maximal (>30-fold) accumulation within 5 h. Increased transcription is partly responsible for this accumulation. This gene encodes a protein of relative molecular mass (M_r) 12,378 which has significant amino-acid homology to platelet factor-4 and β-thromboglobulin, two chemo-tatic proteins released by platelets on degranulation. This IFN-γ-inducible protein may be a member of a family of proteins involved in the inflammatory process.