This study's goal was to assess the safety of tumor-targeted interleukin-12 (ttIL-12) when administered by electrogenetherapy in C3H/HeJ mice by identifying an initial safe dose for human dose escalation schemes, toxicity target organs, markers of toxicity, and toxicity reversibility. Tumor-free mice receiving two doses of 0.45% NaCl, 1 μg ttIL-12 DNA in 0.45% NaCl or 5 μg ttIL-12 DNA in 0.45% NaCl, 10 days apart combined with low-intensity electroporation were compared with non-treatment controls over time. All mice had blood cell counts, serum chemistry profiles, plasma interleukin-12 and IFNγ determinations, necropsy and multi-organ histopathology. Mild treatment-associated changes included electroporation-associated muscle changes that resolved by 30 days; decreased total white blood cell counts and infectious disease in the 5 μg ttIL-12 group, but not in the 1 μg group, and liver changes in ttIL-12 groups that correlated with alanine transaminase levels and resolved by 30 days. Dystrophic cardiac calcification seen in older, 5 μg ttIL-12-treated mice was the only serious toxicity. Based on these results and the lack of any effect on wound healing when combined with surgery, low-intensity electrogenetherapy with ttIL-12 appears to be safe and well tolerated.