Interleukin-2 (IL-2) receptor expression is a feature of T-cell activation and T-cell neoplasia. Expression of the IL-2 receptor in human lymphoid lesions was studied in a series of 166 immunophenotyped cases, including nodal and extranodal reactive lymphoid proliferations (44 cases), low-grade B-cell lymphomas (27 cases), intermediate and high grade B cell lymphomas (42 cases), peripheral T-cell lymphomas (13 cases), Hodgkin's disease (12 cases), histiocytic proliferations (15 cases), nonhematopoietic tumors (16 cases), and miscellaneous lesions (7 cases). Low levels of receptor expression were seen in reactive lymphoid lesions, low-grade B-cell lymphomas, and nonhematopoietic tumors (20%, 7%, and 25% of cases, respectively, with greater than 10% positive cells). High levels of receptor expression were seen in cases of peripheral T-cell lymphoma and histiocytic proliferations (86% and 100% of cases, respectively, with greater than 10% positive cells). Intermediate levels of expression were seen in Hodgkin's disease (including Reed-Sternberg cells) and some cases of intermediate and high-grade B-cell lymphomas (58% and 50% of cases, respectively, with greater than 10% positive cells). IL-2 receptor expression is not confined to T-cell neoplasia, but is also a feature of neoplastic and nonneoplastic histiocytic proliferations, Hodgkin's disease, and some intermediate and high-grade B-cell lymphomas. Biologic and therapeutic implications are discussed.