We undertook a study to determine the specificity of the monoclonal antibody, B-ly-7, for hairy cell leukemia (HCL) by examining the expression in 150 samples from B-cell lymphoproliferative diseases as well as screening for reactivity in a number of other hematologic malignancies. Within the B-cell lineage we found that the expression of B-ly-7 was highly specific for HCL and reacted with all 28 cases examined, as well as with 3 of 9 cases of a variant form of HCL. Cells of other closely related B-cell disorders, prolymphocytic leukemia, and splenic lymphoma with villous lymphocytes were negative. Investigation of the peripheral blood and bone marrow of patients with HCL before and after treatment with alpha-interferon or deoxycoformycin suggests that B-ly-7 may be useful in the assessment of minimal disease after therapy. In addition to HCL, we found that B-ly-7 was positive with cells of three mature, CD4+ T-cell malignancies. In view of the reactivity with malignancies of activated B and T cells, we searched for the expression of B-ly-7 on activated, normal B and T cells and found that B-ly-7 reacted specifically with activated normal peripheral blood CD8+ T cells. B-ly-7 has a number of applications, including the precise classification of mature B-cell neoplasia and the diagnosis HCL and its assessment after treatment. In addition, B-ly-7 recognizes a small subset of T-cell disorders. Its expression on these malignancies and on in vitro activated peripheral blood CD8+ T cells suggests that B-ly-7 detects a lymphocyte activation antigen.