A subpopulation of phenotypically immature (Lyt-2-/L3T4-) thymocytes express receptors for the polypeptide hormone interleukin 2 (IL 2); however, these cells do not proliferate in vitro in response to IL 2. In investigating this phenomenon in greater detail, we observed that the IL 2 receptors (IL 2-R) on freshly isolated immature thymocytes bound IL 2 with about fivefold lower affinity (Kd approximately 100 pM) than IL 2-R on activated mature T cells and T cell lines (Kd approximately 20 pM). Furthermore, in contrast to activated T cells, Lyt-2-/L3T4- thymocytes did not endocytose bound IL 2. When stimulated in short-term culture with a combination of phorbol ester (PMA) and calcium ionophore, Lyt-2-/L3T4- thymocytes proliferated in a largely IL 2-dependent fashion. IL 2-R expression on these activated cells initially disappeared (at 24 hr) and subsequently reappeared (at 48 to 72 hr). Reexpressed IL 2-R on activated thymocytes resembled those on mature T lymphocytes in that they bound IL 2 with high affinity (Kd = 15 to 25 pM) and were capable of endocytosing IL 2. Taken together, these data place certain constraints on the putative physiologic role of IL 2 in intrathymic growth regulation.