The transfer of cytokine genes into cancer cells, resulting in cytokine release directly at the site of tumor growth, has proven effective in inhibiting tumor growth in the absence of any toxic effect. Some cytokines induce tumor suppression even in T-cell-deficient mice, suggesting their potential therapeutic effect in poorly immunogenic tumors; other cytokines induce memory T cells that protect mice from subsequent tumor injection. The effects of cytokine genes transferred into tumor cells are summarized and implications discussed.