The novel observations that intramuscular injection of plasmid DNA preparations could result in myocyte gene expression and induce immune responses to encoded immunogens has generated intense interest in this form of gene therapy. This phenomena can occur with both DNA and RNA reagents, and can be used in immune protection (vaccine) or therapy strategies. Immunization with DNA plasmids has generated protective immunity to a wide variety of pathogens and tumor cells in murine animal models. Immune response has occurred in a broad range of animal species following intramuscular injection of plasmid DNA encoding various immunogens as well as following other routes of administration (intravenous, intradermal, etc). The mechanisms responsible for induction of the immune response are as yet unclear, but responses include antibody production, T-cell proliferation, lymphokine release, generation of cytolytic T cells, and delayed hypersensitivity reactions. Plasmid DNA production and purification methods are relatively easy to standardize, and dual expressing plasmids allow incorporation of immune enhancement molecules or second immunogens. Plasmid DNA encoding nontransforming tumor-associated antigens are in development with a National Institutes of Health-approved protocol for carcinoembryonic antigen in colorectal cancer patients. Transforming tumor-associated antigens (eg, HER2/neu) may be approached with RNA or replicative RNA constructs for immunization. The efficacy of this immune approach will soon be examined in clinical trials in patients with cancer and the acquired immunodeficiency syndrome.