Interleukin-2 (IL-2) plays a central role in the activation and proliferation of lymphocytes that have been primed by antigens. IL-2 plays a pivotal role in the expansion of most T-cells, natural killer cells and B-cells during certain phases of their response. IL-2 gene expression is regulated at the transcriptional level by several activation pathways. Antigen-specific proliferation of helper and cytotoxic T-lymphocytes following stimulation is critically dependent on IL-2 expression, secretion, and binding to receptors for IL-2 induced in an autocrine fashion on the surface of T-cells. Apart from its most important role to mediate antigen-specific T-lymphocyte proliferation, IL-2 modulates the expression of interferon- and major histocompatibility antigens, stimulates proliferation and differentiation of activated B-cells, augments natural killer cell activity and inhibits granulocyte-macrophage colony formation. Alterations in the ability of T-cells to synthesize IL-2 have been observed in physiologic and pathologic states.