This review covers significant developments in the understanding of the biochemistry and clinical pharmacology of Interleukin-2 (IL-2) that were achieved from 1984 through September 1986. These include developments in the molecular biology of IL-2 and its receptors. Human IL-2 was cloned and sequenced by Taniguchi et al. in 1983. The gene for human IL-2 is located on the long arm of chromosome 4. The secondary structure of the gene is predominantly alpha helix. The mature gene product is a 133 amino acid glycoprotein with a molecular weight of 15,420 Daltons. The IL-2 receptor was revealed to be a glycoprotein of 272 amino acids. The mature receptor has a molecular weight of 55,000 Daltons. A more precise understanding of the mechanism of action IL-2, in particular its role in the induction of the IL-2 receptor, and aspects of the control of IL-2 production was also achieved. Metabolic and morphologic studies have revealed that activation of the T-cell antigen receptor renders the cells responsive to IL-2, but does not move them through the cell cycle. Rather, it appears that IL-2 stimulates G1 progression to S phase ie. blastic transformation. During this progression the cellular proto-oncogene c-myb is induced transiently to 6 to 7 times basal levels. The role of IL-2 as a growth factor for several subsets of T cells has been confirmed, and a new role as a growth factor for B cells was defined. Most importantly, IL-2 was shown to be directly mitogenic for and to expand subpopulations of peripheral blood cells, termed lymphokine-activated killer (LAK) cells and tumor-infiltrating lymphocytes (TIL). A number of pathologies of IL-2 production or activity have been defined, including Hodgkin's disease, graft versus host disease, systemic lupus erythematosus, lepromatous leprosy, acquired immune deficiency syndrome, and adult T cell leukemia. Murine and human in vivo studies reviewed here have revealed significant parameters of the therapeutic potential as well as the toxicity of this growth factor. Finally, the modulation of IL-2 receptors on human PBL's by thymosin fraction 5 and thymosin alpha 1 suggests that it might be possible to up-regulate IL-2 receptor expression in certain disease states and thus increase the efficacy of IL-2.