THERE are rare cases in the literature characterized by a severe refractory normochromic normocytic anemia in which there is no reduction in the numbers of leucocytes or platelets. A critical review of these cases of "red cell anemia" shows that in some there are considerable numbers of primitive red cell forms in the bone marrow; these probably represent a maturation arrest of erythropoiesis at an early level. In other cases the bone marrow is characterized by almost complete absence of red cell precursors; these are properly considered cases of true red cell aplasia. The distinction is made clear in the Figure, modified from Whitby and Britton.1 In both types of red cell anemia the aspirated bone marrow appears cellular. In the cases of true red cell aplasia the cellularity is due to normal granulopoiesis and megakaryopoiesis, while in those due to maturation arrest the cellularity is due, in addition,