A plasmid DNA vaccine encoding the human pro-inflammatory cytokine interleukin-12 (IL-12) with potential immunoactivating activity. Upon intramuscular delivery by electroporation of DNA plasmid encoding interleukin-12 INO-9012, IL-12 is translated in cells and activates the immune system by promoting the activation of natural killer cells (NK cells), inducing secretion of interferon-gamma and promoting cytotoxic T-cell responses against tumor cells. This may result in both immune-mediated tumor cell death and the inhibition of tumor cell proliferation.