Lymphokine-dependent T cell proliferation is regulated in part by the cell surface expression of high affinity interleukin-2 receptors (IL-2R). The functional, high affinity form of the IL-2 receptor is comprised of two ligand binding components, IL-2R alpha (Tac, p55) and IL-2R beta (p70/75). In the absence of the other subunit, IL-2R alpha and IL-2R beta bind ligand with only low or intermediate affinity, respectively. The inducible and transient expression of IL-2R alpha regulates the display of high affinity receptors, while IL-2R beta appears to contribute importantly to growth signal transduction. Although the primary structure of both receptor chains has now been elucidated, the mechanism of growth signal transduction through the high affinity IL-2R remains undefined. Of note, IL-2R beta belongs to a novel family of cytokine receptors including the binding proteins for IL-3, IL-4, IL-6, erythropoietin, and granulocyte-macrophage colony-stimulating factor. These various receptors may well utilize a common intracellular signalling pathway.