Cytochemically, nonspecific esterase activity was detected in megaloblasts from three patients with severe untreated pernicious anemia, in megaloblastoid erythroblasts from five patients with chronic erythremic myelosis (DiGuglielmo syndrome), and in normoblasts from a patient with severe untreated iron-deficiency anemia. Nonspecific esterase activity in all of these erythroblasts was inhibited by sodium fluoride. Enzymatic activity could not be detected in normoblasts from normal marrows. Electrophoretically, three bands of nonspecific esterase activity could be visualized in marrow sonicates from the anemic patients and normal persons. All of these bands were inhibited by sodium fluoride. The results demonstrate that electrophoretically and in terms of fluoride inhibition, nonspecific esterases obtained primarily from erythroid precursors in various types of anemias are similar to nonspecific esterases found in normal marrows presumably containing a more heterogeneous population of cells.