Using three murine tumor models, we compared the antitumor efficacy and certain physiological effects of an in vivo interleukin-12 (IL-12) gene therapy protocol and a systemic IL-12 protein therapy protocol. An IL-12 cDNA gene construct was administered in situ into skin tissue via gene gun delivery, and recombinant IL-12 protein was administered subcutaneously at a dose of 1 microgram/mouse/treatment. Both treatment regimes induced a comparable level of regression of established intradermal MethA sarcomas. In B16 melanoma and P815 mastocytoma models, antitumor efficacy of IL-12 protein therapy appeared to be slightly higher than that of IL-12 gene therapy; however, the protein therapy protocol in this comparative study resulted in a high level of mortality of mice. It was also demonstrated that IL-12 gene therapy, in contrast to the IL-12 protein therapy, was not associated with weight loss, splenomegaly, increased Ly6 antigen expression in the spleen, or visible signs of toxicity, such as fur ruffling and lethargy. Moreover, serum levels of interferon-gamma (IFN-gamma) induced in response to IL-12 gene therapy were 300-1000 times lower than those induced by the systemic IL-12 protein administration. Together, these results suggest that gene gunmediated in vivo delivery of IL-12 cDNA may be considered as a safer alternative to IL-12 protein therapy for certain human cancers.