Intratumoral (it) injections of an adenovirus encoding the human interleukin-2 (IL-2) under the control of the RSV (Ad-pRSV-IL-2) or CMV (Ad-pCMV-IL-2) promoter were performed in established mastocytoma P815 tumors in B6D2 mice. Both early and long-term survival were found increased in mice treated with Ad-pCMV-IL-2 as compared with those obtained with Ad-pRSV-IL-2: tumor regression was observed in 30–50% of mice for the former and 5–15% for the latter. Difference in efficacy between the two vectors was directly correlated to the amount of IL-2 produced it between 24 and 48 hours postinjection, which reached 10–20 ng/tumor for Ad-pCMV-IL-2 and 0.3–0.5 ng/tumor for Ad-pRSV-IL-2. In both cases, expression in the tumor was clearly detectable for a period of 7–10 days postinjection. Serum IL-2 was not detectable in mice treated with Ad-pRSV-IL-2, whereas expression peaked at a total of 1–2 ng at 24 hours but declined very rapidly in the Ad-pCMV-IL-2–treated group. Constant production of IL-2 inside the tumor was necessary for successful therapy because it injections of recombinant IL-2 at levels up to 1 μg for five consecutive days did not lead to antitumoral activity. Evidence of induced systemic immunity following Ad-pCMV-IL-2 injections was obtained from rechallenge experiments in which tumor-free mice after treatment rejected a subsequent contralateral injection of a lethal dose of P815 tumor cells and from the observation that regression of nontreated tumors occurred in animals bearing bilateral tumors that were treated it in a single tumor with Ad-pCMV-IL-2. P815-specific cytotoxic T lymphocytes (CTL) were found specifically in spleen cells from cured mice or rechallenged mice but not in control mice. Interestingly, limiting dilution analysis of anti-P815 CTL precursor (CTLp) frequency revealed a significant increase in mice cured of their tumor as compared to that obtained in naive mice or control mice treated or not with Ad-IL-2 but whose tumor was growing. In vivo depletion of T-cell subsets, as well as natural killer cells at the time of it injections with Ad-pCMV-IL-2, demonstrated that both CD8+ T cells and natural killer cells, but not CD4+ T cells, were required for successful therapy. Finally, mice preimmunized with Ad-null viruses were severely compromised in their capacity to eradicate established P815 tumors after Ad-pCMV-IL-2 therapy, at least when neutralizing antibody titers reached a critical level. Cancer Gene Therapy (2001) 8, 321–332