The murine mastocytoma cell line P815 was used as a model to evaluate the effect on its tumorigenic capacity following interleukin-2 (IL-2) gene transfer into the tumor cells using a replication-defective adenovirus vector. The data show that P815 cells infected in vitro with this recombinant adenovirus secreted significant amounts of functional IL-2 as tested on CTL-L2 cells. Furthermore, when injected into syngeneic DBA/2 mice, the tumorigenic phenotype is lost in up to 80% of the animals. The rejection of the infected cells was host dependent, because co-injection at the same site or concomitant injection at the opposite side of the animal with a tumorigenic dose of noninfected P815 cells did not lead to tumor development in 50–70% of the mice. Moreover, protected animals developed a long-lasting state of immunization against the P815 tumor cells and their splenocytes were able to transfer the immunity to syngeneic naïve recipients.