In five patients with acute monoblastic leukemia (AMoL), the ultrastructural localization of myeloperoxidase (MPO) activity was investigated by two methods, one generally used for the detection of MPO and the other for the detection of platelet peroxidase. The MPO-positive rate achieved was lower with the former method than with the later, indicating that MPO is degraded during the fixation of AMoL cells for electron microscopy. If the ultrastructural MPO positivity of leukemic cells varies when different detection methods are used, the possibility of monocytic leukemia should be considered.