The biological function of IL-2 is obtained by binding to the specific interleukin-2 receptor (IL-2R). The IL-2R consists of three non-covalently linked chains, all of which are type I transmembrane proteins and include the α chain (IL-2Rα, p55), β chain (IL-2Rβ, p75), and g chain (IL-2Rγ, p65). The α chain is cleaved from the cell surface via nonspecific proteolysis. IL-2Rα and IL-2Rβγ dimers bind to different residues on the IL-2 protein. The IL-2Rα complex displays low affinity and the IL-2Rαβ complex displays intermediate affinity for IL-2 binding. Both IL-2Rα and IL-2Rαβ complexes are unable to transduce a signal. The IL-2Rβγ complex has intermediate affinity for IL-2 binding and can transduce a signal with a relatively high concentration of IL-2. The IL-2Rαβγ trimer is the high-affinity receptor for IL-2 and can transduce a signal successfully. Many cells are capable of expressing IL-2Rα including the antigen-activated T cells and B cells, and approximately 10% of natural killer (NK) cells, leukemia and lymphoma cells. When produced by activated T cells, the α chain is 10-20 folds in excess of the β and γ chain. A soluble IL-2Rα can be detected in tissue culture media of IL-2R+ cells and in the serum of experimental animals and humans undergoing an immune response. The major biological activities of IL-2R include promoting the proliferative expansion of T cells and NK cells upon activation, promoting the persistence of antigen-selected memory T cells, and promoting homeostasis of the immune system after it has successfully responded to an antigen. However, the biological activity of soluble IL-2Rα is unclear. It has been reported that elevated IL-2 sRα level is accompanied by increased T and B cell activation and immune system activation as observed in rheumatoid arthritis, systemic lupus erythematosis (SLE), some leukemias and lymphomas. Because of its low affinity, IL-2 sRα would be expected to be an inhibitor of IL-2. This IL-2 sRα ELISA is a ready-to-use 4.5-hour solid phase immunoassay capable of measuring IL-2 sRα levels in serum, plasma, cell culture supernatant, and other biological fluids in the range of 0 to 2000 pg/mL. This assay has shown no cross-reactivity with other cytokines tested, and is expected to be used effectively for further investigations into the relationship between IL-2 sRα and the various conditions mentioned.