Cytokines are immunologically active but antigen-nonspecific substances other than immunoglobulins. They have several common characteristics as follows: 1) They are glycosylated polypeptides. 2) They are biologically active at very low concentrations (10(-10) - 10(-12) M). 3) They exert their effects by binding to their receptors on the cell surface. 4) Their effects are target cell-specific. 5) One cytokine has multiple biological activities. 6) Different cytokines often show the same effect. 7) They interact on various target cells forming a complex network. Successful gene cloning of various cytokines has provided us a large quantity of pure proteins which have been used not only in the basic studies but also in the trial of the treatment of cancers and immunological disorders. Interleukin-2 and its receptor system is one of the most extensively-studied cytokine systems. The progress of the basic research of IL-2/IL-2 receptor system and its application to the diagnosis and treatment of various diseases are reviewed.