Nonobese diabetic (NOD) mice develop insulitis and diabetes through an autoimmune process. Since TGF-β1 down-regulates many immune responses, we hypothesized that TGF-β1 could prevent disease in NOD mice and that there would be several advantages to cytokine delivery by a somatic gene therapy approach. We opted for im injection of a naked plasmid DNA expression vector encoding murine TGF-β1 (pCMV-TGF-β1). Treatment with pCMV-TGF-β1 resulted in the retention and expression of the vector in muscle cells, associated with a considerable elevation in the plasma levels of TGF-β1, that was not observed in control vector-treated mice. The levels of TGF-β1 produced were sufficient to exert immunosuppressive effects. Delayed-type hypersensitivity responses were suppressed, and autoimmunity-prone NOD mice were protected from insulitis and diabetes in models of cyclophosphamide-accelerated and natural course disease. In pCMV-TGF-β1-treated mice, pancreatic IL-12 and IFN-γ mRNA expression was depressed, and the ratio of IFN-γ to IL-4 mRNA was decreased, as determined by semiquantitative reverse-transcription PCR. In contrast, NOD mice injected with a vector encoding the proinflammatory cytokine IFN-γ developed diabetes earlier. Intramuscular administration of cytokine-encoding plasmid vectors proved to be an effective method of cytokine delivery in these mice, and altered autoimmune disease expression.