1) The free amino acids in the serum of patients with hypoplastic anemia showed a low amino acid fractionation pattern with a marked decrease of glycine and alanine and the presence of unidentified X substances. 2) The conjugated amino acids from the same source demonstrated a similar fractionation pattern with reduction of glycine and alanine and accompanying X substances. 3) The free amino acids in the urine of patients with hypoplastic anemia were generally increased in comparison to those of normal individuals. The urinary excretion of glycine was particularly prominent. 4) From these finding it is concluded that amino acid metabolism in hypoplastic anemia is unique. 5) Injection of X substances into rabbits induced a temporary, slight anemia. 6) The author believes that derangement of amino acid metabolism has something to do with the development of hypoplastic anemia.