The high-affinity IL-2-R complex is composed of at least two distinct IL-2-binding subunits, including p55 (Tac, IL-2-R alpha) and p70 (IL-2-R beta). Using a radiolabeled mAb specific for the p55 receptor subunit and cells expressing a homogeneous population of high-affinity binding sites, we demonstrate that p55 is co-internalized with p70 after IL-2 binding to the receptor complex. Endocytosis of p55 depends upon the presence of IL-2 in a form capable of effectively interacting with the p70 subunit. Whether IL-2 is required for high-affinity receptor assembly or triggering of the internalization of preassembled receptors remains unresolved. Together, these findings support the existence of a stable, high-affinity human IL-2-R membrane complex composed of at least the p55 and p70 receptor subunits and IL-2.