To see if immunohistochemistry can be used on routinely processed bone marrow biopsies for diagnostic purposes, 73 biopsy specimens, fixed in sublimate-formaldehyde, decalcified in an acetic acid-formaldehyde mixture, and embedded in paraffin, were studied with a panel of antibodies. The specimens included "normal," lymphomatous, and myeloproliferative disorders as well as some biopsies with metastatic carcinoma. The results show that the different cell lines and their localization in the bone marrow can be easily identified and quantitative and qualitative changes can be assessed. Megakaryopoiesis is identified with Factor VIII-related antigen and Ulex europaeus agglutinin (UEA); myelopoiesis stains with MT-1, elastase, Leu M-1, LN-2, LN-3, HECA 452, and 115D8; and in myeloproliferative conditions, myeloblasts and promyelocytes stained with leukocyte common antigen (LCA). Erythroid cells stained with UEA, glycophorin A, and LN-1. Lymphocytes were marked with LCA, MB-2, and LN-2. Plasma cells were stained best with immunoglobulin light chain antisera; only occasional reactivity with LCA and 115D8 was observed. Carcinomas all reacted with MB-2; occasional reactivity with 115D8, HECA 452, LN-1, LN-2, MT-1, and Ber H-2 was seen. A small panel of selected antibodies, such as UEA, Leu M-1, and LCA, and the immunoglobulin light chain antisera can be very helpful in bone marrow diagnosis and would cover most indications.