Summary. There exist several cytochemical reactions known as ''peroxidase'' reactions or ''oxidase reactions'' employed to detect myeloperoxidase (MPO) activity in leukocytes. These reactions like those of Armitage, Graham, Graham-Knoll, Sato-Sekiya, Washburn, Winkler and other authors, very often do not give positive results when dealing with the most immature forms of myelogenous cells, namely myeloblasts. Qualitative and quantitative tests of the reagents used in these reactions were carried out. The demonstration and localization of the enzyme activity by means of these reactions depend primarily on the correct choice of the concentration of the reagents used, as well as on the concentration of myeloperoxidase contained in granulocytic cells. Benzidine base and hydrogen peroxide have been employed in quantities necessary to develop the cytochemical myeloperoxidase reaction in optimal biochemical conditions. The compound developed by these cytochemical methods produces a greenish-yellow colour in the cytoplasm of myelogenous cells. Two new methods are reported: one for bone marrow smears and the other for peripheral blood smears. With the former positive results have been obtained in myeloblasts, which promises to be helpful in the differential diagnosis of myeloblastic leukemia from other types of leukemia.