1. Erythrocyte and plasma cholinesterase activities of the blood were determined by Michel’s potentiometric method in 20 healthy individuals and in 20 patients ill with various blood dyscrasias.
2. The mean erythrocyte cholinesterase activity in the blood of healthy individuals was 0.71 ± 0.11 Δ pH/hr. and the mean plasma cholinesterase activity was 0.98 ± 0.17 Δ pH/hr.
3. Low erythrocyte cholinesterase activity was found in the blood of patients ill with pernicious anemia or ill with hypoplastic anemia.
4. High erythrocyte cholinesterase activity was found in the blood of patients suffering with anemia secondary to hemorrhage and of patients ill with pernicious anemia or macrocytic anemia of nontropical sprue in early remission.
5. The plasma cholinesterase activity of blood from patients ill with blood dyscrasias usually reflected the general clinical status of the patients and, in general, was related to the albumin concentration in the serum.
6. The low erythrocyte cholinesterase activity increased following multiple blood transfusions and during a reticulocyte response in a case of hypoplastic anemia, whereas the plasma cholinesterase activity remained unchanged.
7. The erythrocyte cholinesterase activity in patients ill with pernicious anemia or macrocytic anemia of nontropical sprue increased rapidly during therapy beginning with the reticulocyte response and continuing during the period of increased erythropoietic activity. The plasma cholinesterase activity increased slowly parallel to improvement in hematologic and clinical status.
8. The possible relationships between alterations in erythrocyte cholinesterase activity in blood dyscrasias and reticulocytosis and erythropoiesis are discussed.