Hemolytic disease of the newborn (HDN) arising from MNSs incompatibility is rare, with few reports of prolonged anemia and reticulocytopenia following HDN. We report the younger of 2 male siblings, both of whom had anti-M-induced HDN and anemia persisting for over a month. Peripheral reticulocytes remained inappropriately low for the degree of anemia, and they needed multiple red cell transfusions. Viral infections were ruled out. Corticosteroids were given for suspected pure red cell aplasia. Anemia and reticulocytopenia subsequently improved. Colony-forming unit erythroid assay revealed erythropoietic suppression of M antigen-positive erythroid precursor cells cultured with maternal or infant sera containing anti-M. In conclusion, maternal anti-M caused HDN and prolonged anemia by erythropoietic suppression in 2 siblings.