Interleukin 15 is a newly discovered cytokine that shares biological activities with IL-2 and, like IL-2, is a member of the four-helix bundle cytokine family. We have shown that IL-15 shares components of the receptor for IL-2: the α chain of the IL-2R is not required, but both the β and γ chains are needed for EL-15 mediated bioactivities. A defect in IL-15 signaling may therefore contribute to the phenotype of X-linked severe combined immunodeficiency in humans, resulting from mutations in the common γ chain. Differential ability of cells to bind and respond to IL-2 and IL-15 suggested the existence of an additional IL-15 specific receptor component. We identified an IL-15 specific binding protein (IL-15Rα) on a murine T cell and isolated the corresponding cDNA. The IL-15Rα is not a member of the hematopoietin receptor superfamily, but is structurally related to the α chain of the IL-2R. Differences in the expression pattern of EL-15 and its receptor compared to the EL-2 system suggest unique in vivo roles for IL-15. J. Leukoc. Biol. 57: 763–766; 1995.