In vivo transfection of established tumors with immunostimulatory genes can elicit antitumor immunity. Therefore, we evaluated the safety and efficacy of intratumoral injections of a bacterial superantigen with a cytokine gene in dogs with malignant melanoma, a spontaneous and highly malignant canine tumor. 26 dogs with melanoma were treated with lipid-complexed plasmid DNA encoding staphylococcal enterotoxin B and either GM-CSF or IL-2. Dogs were evaluated for treatment-associated toxicity, tumor responses, immunologic responses, and survival times. The overall response rate (complete or partial remissions) for all 26 dogs was 46% (12 of 26), and was highest in patients with smaller tumors. Toxicity was minimal or absent in all dogs. Injected tumors developed marked infiltrates of CD4+ and CD8+ T cells and macrophages, and tumor regression was associated with development of high levels of antitumor cytotoxic T lymphocyte activity in peripheral blood lymphocytes. Survival times for animals with stage III melanomas treated by intratumoral gene therapy were prolonged significantly compared with animals treated with surgical tumor excision only. Thus, local tumor transfection with superantigen and cytokine genes was capable of inducing both local and systemic antitumor immunity in an outbred animal with a spontaneously developing malignant tumor.