Detailed hematologic studies were carried out in an infant with hemolytic disease of the newborn following exchange transfusion who manifested a high titer of anti-D antibody in the serum and evidence of impaired erythropoiesis. The relation between falling antibody titer and resumption of erythrocyte production supports the hypothesis that the antibody may act against erythrocyte precursors. The various stages of involvement of the erythron by antibodies are discussed.