Interleukin 2 (IL-2) mediated signalling results from ligand binding and subsequent heterodimerization of IL-2r beta and gamma c. The high-affinity IL-2 receptor (IL-2r) is a heterotrimer comprised of the IL-2r alpha, IL-2r beta and gamma c subunits. Whereas human IL-2 effectively binds to either human or murine lymphocytes, murine IL-2 binds with markedly higher affinity to murine receptor complexes than to human complexes. Using cell lines stably expressing heterotrimeric IL-2r that vary in the species origin of individual subunits, we have demonstrated that IL-2r alpha is primarily responsible for the species specificity of IL-2 binding. Studies of ligand binding to the low affinity receptor demonstrated that IL-2r alpha displays a similar species preference to the heterotrimeric complex. Moreover, differences in ligand binding are reflected in differences in proliferation. A cell line expressing human IL-2r alpha and IL-2r beta along with murine gamma c vigorously proliferated only in response to human IL-2 at low doses, while both human and murine IL-2 stimulated proliferation of a cell line containing murine IL-2r alpha (as well as human IL-2r beta and murine gamma c). Therefore, IL-2r alpha is the chain primarily responsible for the species specificity of ligand binding.