To investigate the effect of plasmid coding interleukin-10 (IL-10) DNA on the development of autoimmune diabetes induced by multiple low doses of streptozotocin (STZ) in mice. Injection of STZ (40 mg/kg, ip) was given daily for five consecutive days. pcDNA3-IL-10 plasmid (IL-10-treated group) or pcDNA3-null plasmid (pcDNA3-null-treated group)(100 µg DNA once a day) were injected into skeletal muscles of mice on d 1 and d 14. Blood glucose concentration was measured. After mice were killed on d 28, serum IFN-γ level was measured by ELISA, and pancreatic IL-1β and TNF-α mRNA expression was detected by semi-quantitative reverse-transcription PCR (RT-PCR). The number of CD4+ and CD8+ lymphocytes from spleen was detected using FACS. In addition, pancreatic histology was measured for determination of insulitis grades. Treatment with pcDNA3-IL-10 resulted in the retention and expression of the vector in skeletal muscle, associated with a considerable elevation in the plasma level of IL-10, which was not observed in pcDNA3-null-treated mice. In IL-10-treated diabetic mice induced by STZ, delay-type hypersensitivity responses were suppressed and the glucose level was greatly lower on d 14, 21, and 28 than pcDNA3-null-treated group (P< 0.05 or P< 0.01). On d 21 and 28 the incidence of diabetes was 33.3% and 40.0%, respectively, which was markedly lower than that of pcDNA3-null-treated group (P< 0.05). In IL-10-treated mice pancreatic IL-1β and TNF-α mRNA expression was depressed, and serum IFN-γ concentration and the number of spleen CD4+ or CD8+ lymphocytes were decreased on d 28. The insulitis grades of IL-10-treated mice were lower than that of pcDNA3-null-treated group (P< 0.01). Systemic administration of IL-10 plasmid DNA can alleviate insulitis of experimental autoimmune diabetes in mice and reduce incidence of diabetes.