Summary. Esterase activity was investigated in blood cells of peripheral blood and bone marrow in healthy subjects and in various pathological conditions. As substrates were used: α‐naphthyl acetate, naphthol AS‐D acetate and naphthol AS‐D chloroacetate. Fast blue B and fast garnet G. B. C. were used as coupling agents.The distribution of AS‐D acetate esterase in different blood cells was described. Strong α‐naphthyl acetate esterase activity was found in monocytes, reticulum cells, megakaryocytes and in monoblasts in cases of monocytic leukaemia. Weak activity was shown in thrombocytes, and in lymphocytes.With naphthol AS‐D chloroacetate as substrate strong enzymatic activity was demonstrated in the cells of the neutrophilic myelocytic series. The activity was strong in the more mature cells. Blasts from acute myelocytic leukaemia showed enzymatic activity.The use of α‐naphthyl acetate and naphthol AS‐D chloroacetate as substrates proved to be of help in differentiating between the three types of acute leukaemia.In cases with megaloblastic erythropoiesis only strong α‐naphthyl acetate esterase activity was demonstrated in the megaloblasts. Successful treatment of pernicious anaemia with B12, resulted in disappearance of megaloblasts and cessation of the strong esterase activity. There seems to be a correlation between strong naphthyl acetate esterase activity and the presence of megaloblasts. An explanation of this ph