The cytokine interleukin 2 (IL 2) has proved to be an effective agent in immunotherapy of human cancers. Natural killer cells and cytotoxic T-lymphocytes are activated via membrane receptors by IL 2. IL 2-activated natural killer cells and cytotoxic T-lymphocytes develop high cytolytic activity against a variety of tumor cells in vitro. After extensive animal experiments, IL 2 was used in humans either alone or in combination with lymphokine-activated killer cells or tumor-infiltrating lymphocytes. Its application in humans is still experimental, although initial results are very promising. Further studies are necessary to establish treatment protocols with high therapeutic efficacy and tolerable side effects.