We report on long-term delivery of an interferon-γ (IFNγ) inhibitory protein by intramuscular (im) gene therapy. IFNγ is a cytokine that plays an important role in many inflammatory disorders, including autoimmune insulin-dependent diabetes mellitus (IDDM) in NOD mice and (in various strains) multiple low-dose streptozotocin (STZ)-induced diabetes (MDSD). By cDNA insertion into plasmid VICAL VR-1255 we constructed an expression vector encoding a soluble IFNγ receptor/IgG1 heavy chain (all murine) fusion protein (IFNγR/IgG1). This protein is secreted as a homodimer and neutralizes IFNγ in vitro. We show that im injections of this vector as naked DNA in mice results in secretion of IFNγR/IgG1, with serum levels exceeding 100 ng/ml for months after treatment. These levels are sufficient to neutralize IFNγ in vivo, and to prevent either MDSD or cyclophosphamide (CYP)-accelerated diabetes in NOD mice, which are both characterized by systemic release of IFNγ. In these diseases gene therapy considerably reduces inflammation in the islets of Langerhans (insulitis). Also, circulating IFNγR/IgG1 blocked IFNγ-enhanced nitric oxide production by peritoneal macrophages. The fusion protein is constructed from nonimmunogenic self elements, avoiding a neutralizing immune response and making it suitable for prolonged therapy of numerous inflammatory disorders.