Abstract
1. Human myeloid cells in the peripheral blood and bone marrow contain a very strong esterase activity which can be demonstrated histochemically by the use of naphthol AS-D chloroacetate as substrate.
2. As first pointed out by Gomori, this esterase activity is found not only in segmented but also immature myeloid cells. However, stem cells, cosinophilic and basophilic granulocytes, lymphocytes, monocytes, plasma cells, megakaryocytes, adult red cells and red cell precursors do not stain with this method.
3. Histocytes and mast cells are strongly positive for this esterase activity.
4. No differences in esterase activity have been noted thus far between neutrophilic leukocytes in normal individuals and in patients with chronic myclogenous leukemia or other diseases.
5. The leukocyte esterase activity is abolished by various well known inhibiting agents. Activity in young myeloid cells in non-leukemic bone marrow smears and in the peripheral blood of patients with myeloid leukemia is inhibited less than that in adult segmented neutrophils.
6. The functions of this esterase in leukocytes are unknown, but further studies of this enzyme seem warranted.