1. Activity-pS curves were determined experimentally for the cholinesterase of the erythrocytes in various human anemias. 2. The data were analyzed to give experimental values for the apparent dissociation constants, K8 of the active enzyme-substrate complex (ES) and K2 of the inactive complex (ES2), and the theoretic limiting velocity V. 3. The analysis showed that the enzyme in the cells of anemia is the same as that in normal cells. The abnormally high enzymic activity (initial velocity at a single substrate concentration) known to occur in various anemias and the change from low activity in pernicious anemia in relapse to high activity during treatment are attributable to changes in concentration of the enzyme. 4. The enzyme in the red cells of pernicious anemia in relapse was not activated by folic acid or liver extract. 5. It was shown experimentally that the reticulocytes and young cells of normal blood and of pernicious anemia during treatment contain much higher concentrations of enzyme than old cells. The increased concentration persists for some time after the reticulum has disappeared. 6. The relative concentration of enzyme in the red cells is a more sensitive indicator of hyperactive hematopoiesis than the reticulocyte count. 7. Evidence is presented that failure of elevation of enzymic concentration in severe anemia is associated specifically with suppression or malfunction of the hematopoietic system. 8. It is believed that the relative concentration of enzyme can be used as a test of bone marrow function in severe anemias. It is hoped that further investigation may result in a useful test of wider application.