Intratumoral (i.t.) injection of a plasmid DNA vector encoding the murine interleukin-2 (IL-2) gene was used to treat established renal cell carcinoma (Renca) tumors in BALB/c mice. Tumor regression was observed in 60-90% of mice that were injected i.t. for 4 days with IL-2 plasmid DNA complexed with the cationic lipid DMRIE/DOPE ((+/-)-N-(2-hydroxyethyl)-N,N-dimethyl-2,3-bis(tetradecyloxy)-1-propa naminium bromide/dioleoylphosphatidylethanolamine). The mice remained tumor-free until the conclusion of the study, which was 4 months after tumor challenge. In a rechallenge experiment, mice that were rendered tumor-free for 6 months by IL-2 plasmid DNA treatment rejected a subsequent challenge of Renca cells but could not reject a challenge with the unrelated, syngeneic CT-26 tumor. Spleen cells from cured mice contained Renca-specific cytotoxic T lymphocytes, and adoptive transfer of mixed lymphocyte cultures into naive mice at 2 days after challenge with Renca cells prevented tumor growth. In vivo depletion of T-cell subsets at the time of i.t. injection with IL-2 plasmid DNA demonstrated that CD8+ T cells, but not CD4+ T cells, were the primary effectors of the antitumor response.