A good correlation exists between zinc content and carbonic anhydrase activity of the red blood cells under all conditions studied, including anemia and polycythemia. In almost all patients with anemias other than pernicious anemia, both zinc and carbonic anhydrase levels were lowered in parallel fashion. These changes were proportional to decreases in hematocrit and hemoglobin levels and erythrocyte counts so that both zinc and carbonic anhydrase values per unit of RBC were in the normal range. In a few instances of anemia associated with leukemia and in one of sickle cell anemia, neither zinc content nor carbonic anhydrase activity was decreased in proportion to the anemia; in these cases the zinc and carbonic anhydrase levels per unit of blood were both elevated to the same degree.

Patients with pernicious anemia showed no decrease in absolute values for zinc and carbonic anhydrase activity in spite of marked lowering of hematocrit and hemoglobin levels and of erythrocyte count. Accordingly, both zinc concentration and carbonic anhydrase activity per unit of blood were elevated, often to a marked degree. These increases were parallel, varying inversely with the degree of anemia; when they regressed under treatment, both did so at the same rate.

There are no methods available for estimating carbonic anhydrase concentration; all methods now in use measure only the activity of the enzyme. It is suggested that zinc concentration could be used as an indicator of carbonic anhydrase content of the red blood cells.

ACKNOWLEDGMENTS Drs. Joseph C. Aub and Ira T. Nathanson were kind enough to refer several patients for study. Dr. Byrl J. Kennedy was most helpful in regard to obtaining samples of blood. The technical work was performed by Miss Mary Lou Roney, Betty Hickey and Marion Taylor.