In a study of 55 patients with either acute lymphoid leukemia (ALL; 25 cases) or acute myeloid leukemia (AML; 30 cases), paraffin-embedded bone marrow particle sections were examined with a panel of monoclonal and polyclonal antibodies reactive toward lymphoid and myeloid-associated antigens, using the alkaline phosphatase-anti-alkaline phosphatase (APAAP) technique. All cases were previously classified according to the French-American-British (FAB) Co-operative Group, and cases of ALL were immunophenotyped by flow cytometry. Results indicated that myeloid-associated antibodies (Mac 387, KP 1 [CD68], antielastase, antilactoferrin, and antilysozyme) did not react with any case of ALL, M1-AML, or M6-AML, whereas at least one of these antibodies reacted with 20 of 21 (95%) cases of M2, M3, M4, and M5-AML. Anti-glycophorin C marked cases of M6-AML, whereas anti-CD3 labeled T-cell ALL. None of the antibodies tested specifically identified cases of B-cell ALL. The authors conclude that use of a selected panel of antibodies on paraffin-embedded bone marrow particle sections may be of value in the diagnosis and immunophenotypic classification of many cases of acute leukemias.