Since the last workshop on human leukocyte differentiation antigens, there are 14 well defined cluster-designated (CD) antigens which characterize myelomonocytic cells. Of these, 5 are potentially useful for myeloid leukemia typing (i.e. CD13, CD14, CD15, CD33, CD36) because they are cell lineage-specific and also expressed on immature cells. However, the reactivity of monoclonal anti-CD antibodies, directed against these antigens, with myeloblastic leukemia cells was found to be quite low. We produced monoclonal antibodies against myeloperoxidase. These antibodies react also with promyeloperoxidase, synthesized in HL-60 cell line cells. Monoclonal antimyeloperoxidase was found to be the most sensitive reagent to diagnose acute myeloid leukemia, even more sensitive than cytochemical stains (Sudan black, myeloperoxidase).