Abstract
When the marrow from a patient with red cell aplasia was incubated in vitro a marked increase in heme synthesis occurred. Heme synthesis was further increased by the addition of erythropoietin. The patient’s plasma contained both a γG-antibody to erythroblast nuclei and a γG-fraction that inhibited heme synthesis. After the patient was treated with cyclophosphamide, the antinuclear antibody and the inhibitor to heme synthesis disappeared and erythropoiesis was normal. These experiments confirm our previous study and indicate that red cell aplasia may be due to an antibody to marrow erythroblasts.