Interleukin 2 (IL-2), or T-cell growth factor (TCGF), represents the first identified, fully-characterized, purified human interleukin. It is produced mainly by T helper (CD4+) lymphocytes, stimulates cell-mediated immune responses, controls growth and differentiation of B lymphocytes, and intensifies proliferation and activity of all cytotoxic cell clones. IL-2 is a growth factor in vitro and a mediator of self-tolerance in vivo, and therefore interests tumor immunotherapy investigators. The role of IL-2 in the cell cycle of neoplastic cells remains unclear. IL-2 inhibits growth of certain human tumor cells while proliferation of other cells remains intact or is even stimulated. Decreased IL-2 production is often observed in the more advanced clinical stages of human tumors, which provides rational for inclusion of recombinant IL-2 in the immunotherapy for some tumors. On the other hand, tumor cells themselves may produce IL-2 and promote tumor growth. This article summarizes the current physiological role of IL-2 and its role in the pathogenesis of select human diseases. Many papers (including reviews) pertain to the IL-2R receptor. The soluble form of the alpha subunit of the IL-2 receptor (sIL-2Ralpha) is elevated in most proliferative disturbances of the hematopoietic system and in many solid tumors. Special reference to the most important discoveries and our own experience in intracellular detection of IL-2 and IL-2Ralpha are included. IL-2 properties, cellular sources, and targets, including data on its expression in pathological conditions, continue to be supplemented. Attempts to treat tumors are also discussed, using modified varieties of therapy that use IL-2 itself and/or its receptor.