In this review, salient molecular, biochemical and functional features of human interleukin 2 (IL-2), its membrane receptor, and its clinical relevance are outlined. We also describe experimental systems, where observed biological or pharmacological effects of IL-2 could be applied to corresponding clinical situations. In particular, IL-2 has been intensively studied in the context of cancer therapy. We discuss the rationale for the use of IL-2 in cancer treatment and our experience in this area. A better understanding of the IL-2 system and, specifically, the nature of signals transduced through it will allow us to manipulate the immune response in a variety of different ways, resulting in new approaches to investigation of immune responsiveness in general. This may have a profound impact on clinical medicine.