[en] In 11 patients with megaloblastic anemia, transfusion of packed erythrocytes or washed erythrocytes invariably resulted in a decline in plasma iron concentration to a range of 20-90 µg/dl (3.6 - 16 µmol/l) after 36 or 48 hours. The same phenomenon was observed in two of six cases of ineffective erythropoiesis without megaloblastosis and in none of five cases of aplastic anemia. The observed changes did not result from a specific hematinic response or from iron uptake by a non-erythroid compartment. In megaloblastic anemia, alteration in marrow function in response to transfusion was reflected by plasma iron kinetics and serum lactate dehydrogenase values, which indicated marked reductions in both marrow hyperplasia and ineffective erythropoieseis. Transfusion in megaloblastic anemia was also responsible for a 50% reduction in platelet count after 2 to 6 days. The significance of these changes is discussed.