Since the discovery by Castle and Minot1of the role played by substances present in liver in achieving a clinical and hematological remission in pernicious anemia, great advances have been made toward elucidating the metabolic disturbances leading to the production of macrocytic anemias associated with a megaloblastic bone marrow. There is considerable experimental evidence to indicate that folic acid, folinic acid, and vitamin B12are enzymes involved in the manufacture of nucleic acid. The former two act in the early phases to form purines and pyrimidines, which with ribose and desoxyribose form thymidine. Vitamin B12acts at this level to convert thymidine into nucleic acid. A deficiency of any of these factors is capable of producing a megaloblastic bone marrow.2Experimentally a megaloblastic anemia has been produced by May3and associates in monkeys fed a diet deficient in folic acid and ascorbic acid. Jones,4Davidson,