An immunotherapeutic agent consisting of a plasmid DNA encoding human Interleukin-2 (IL-2) complexed with a cationic lipid, 1,2-dimyristyloxypropyl-3-dimethyl-hydroxyethyl ammonium bromide/dioleyl-phosphatidyl-ethanolamine (DMRIE/DOPE), in a 5:1 ratio. Due to the lipophilic nature of this cation liposome complex, this gene transfer system may improve the efficiency of introducing the IL-2 gene into the cells, thereby increasing the production of IL-2 and inducing an immune response.