Interest in interleukin-2 results from the fact that it works in vivo on the cell-mediated arm of the immune response and in vitro it is effective on naive lymphocytes. Although it has no direct cytotoxic effect on neoplastic cells, it does help mediate the destruction of tumor cells. Current clinical trials are attempting to determine optimum dose and duration of treatment, since the toxicity of IL-2 appears to increase in proportion to dosage. The authors summarize our knowledge to date.