1) Administration of amino acid preparations in patients with hypoplastic anemia resulted in some amelioration of the low fractionation pattern of serum free amino acids, but glycine and alanine remained decreased and X substances persisted. 2) Although hematological remission was seen following steroid therapy in the majority of patients with hypoplastic anemia, the amino acid fractionation pattern characteristic of this disease was unaltered with persistance of X substances. 3) The fractionation pattern of serum free amino acids was not modified by splenectomy in hypoplastic anemia.