The expression of interleukin-2 (IL-2) receptor (CD25) has been investigated in 165 cases of hematologic neoplasia by alkaline phosphatase-antialkaline phosphatase (APAAP) labeling of cell smears using two monoclonal anti-IL-2 receptor antibodies (anti-Tac and DAKO-IL2-R). IL-2 receptor was found in 2 of 16 (13%) T-cell malignancies (1 T-ALL, 1 Sezary syndrome). In contrast, among B-cell disorders, IL-2 receptor was expressed in the majority of hairy cell leukemia cases (14 of 16, i.e., 88%) and in a smaller proportion of cases of common acute lymphoblastic leukemia (7 of 44, i.e., 16%), B-cell chronic lymphocytic leukemia (8 of 20, i.e., 40%), B-cell prolymphocytic leukemia (3 of 5), and B-cell lymphoma (3 of 8). In addition, IL-2 receptor was present on the neoplastic cells in some cases of acute myeloid leukemia (6 of 44, i.e., 14%) and acute undifferentiated leukemia (4 of 7). IL-2 receptor (Tac antigen) expression is thus found across the spectrum of hematologic neoplasms and is not restricted to T-cell disorders. The explanation for the expression of the IL-2 receptor by such a variety of different hematologic malignancies remains unclear.