To evaluate the feasibility of acute leukemia typing on routinely processed bone marrow biopsy specimens, 72 cases of previously established acute leukemia covering the spectrum of 17 known subtypes were studied immunohistochemically. Most leukemic myeloblasts were positive for myeloperoxidase in 16 (84%) of 19 cases of acute myeloid leukemia, M1-M4, and M6. Most leukemic cells in 11 of 12 M4 and M5 cases were positive for CD68 (PG-M1). All six M6 cases stained with hemoglobin. Leukemic megakaryoblasts in three of four M7 cases were positive for factorThe diagnosis and classification of acute leukemia usually are made on the basis of examination of WrightGiemsa-stained smears of bone marrow aspirate or peripheral blood (or both) in conjunction with cytochemistry, immunocytochemistry, or immunophenotyping by flow cytometry. Bone marrow trephine biopsy is helpful in evaluating cellularity, fibrosis, megakaryocyte clustering, granulomas, and metastatic tumors. It rarely is used alone for the initial diagnosis and classification of acute leukemia. However, in certain situations, smears of peripheral blood or marrow aspirate are inadequate or not available for study, for example, a dry tap because of bone marrow fibrosis, hypocellular marrow aspirate (hypocellular acute leukemia), 1 technical problems, or the unexpected finding of acute leukemia. Thus, pathologists may need to rely on the paraffin blocks of bone marrow for the diagnosis and even classification of acute leukemia. Cell lineage-specific antibodies that can be applied to paraffin sections have become available during the last few years. Most of these antibodies have been evaluated individually for various types of acute leukemia. The number of reports on the use of these newly developed, commercially available cell lineage-specific antibodies to study immunohistochemically the entire subset of acute leukemia is small.2 ' 3The aims of this study were to evaluate how accurately subtyping of acute leukemia can be made on the basis of immunohistochemical studies performed on paraffin sections and to establish a practical costeffective immunohistochemical study for the diagnosis of the spectrum of acute leukemia.
MATERIALS AND METHODSSeventy-two cases representing the entire subset of acute leukemia with cell lineage previously typed on the basis of cytochemical (Ml-5, AEol, ABaL) or immunocytochemical (M6-7) studies or immunophenotyping by flow cytometry (acute lymphoblastic leukemia [ALL], M0) of bone marrow aspirates were retrieved from the files of the Mayo Clinic, Rochester, Minn. These cases had been diagnosed and classified according to the French-American-British (FAB) C o o p e r a t i v e G r o u p c r i t e r i a .4 -8 L e u k e m i a s not defined by the FAB group were classified by other proposed criteria as acute eosinophilic leukemia 410