We produced anti-myeloperoxidase(MPO) monoclonal antibodies(moAb) to detect MPO sensitivity in leukemic cells by visual light microscopy. We got seven kinds of moAbs which showed different properties to MPO. These antibodies reacted to MPO in immature and mature neutrophils, monocytes and HL-60(human myeloid leukemic cell line) by immunochemical detection using the avidin-biotin complex(ABC) staining method, but the amounts of immune complex products were significantly different at each moAb. MPO2, MPO4, MPO9 and MPO13 moAbs blocked MPO activities but MPO5 showed no such inhibition. Most of the moAbs reacted to 59kD MPO moAb using the western blotting method, while MPO4 and MPO5 reacted to both 81kD and 89kD MPO precursors. This data suggests that these moAbs are useful to demonstrate a small amount of MPO in leukemic cells.