The host-immune response against adenoviruses forms a major obstacle for their use as gene therapy vectors for treatment of genetic defects. None the less, they are the preferred vectors for in vivo gene transfer in experimental gene therapy protocols for cancer. In this article we demonstrate the antitumor efficacy of adenovirus-mediated transfer of human interleukin-2 cDNA in the rat-CC531 model for hepatic metastases of colorectal cancer: intratumoral administration of 10 8 plaque-forming units of the hIL-2-expressing adenoviral vector, AdCAIL-2, resulted in a cessation of tumor growth in 80% of the injected tumors. In control groups receiving AdCnull, a vector with the same viral backbone, but lacking transgene expression, none of the tumors responded. However, intratumoral treatment with this vector significantly enhanced tumor regression induced by systemic IL-2 protein treatment, which was used as a positive control. In addition we show, by performing delayed-type of hypersensitivity assays, that AdCnull when injected intratumorally enhances recognition of tumor antigens by T lymphocytes to the same extent as intratumoral treatment with the IL-2-expressing vector. The replication-deficient adenoviruses appear to have a therapeutic advantage in cytokine-mediated immunotherapy: even adenovirus vectors that do not express a transgene, show adjuvant activity and stimulate an antitumor immune response.