Hematological and histological studies of the rabbit brachydactyly mutant were made to clarify the question of the blood abnormality that had previously been implicated in the teratogenic process leading to congenital amputation. The hemoglobin of fetal and adult br/br rabbits, studied by electrophoresis and cyanmethemoglobin assay, exhibited no differences from that of controls. The br/br fetuses were polycythemic and severely macrocytic. Fetal limb vessels showed thrombosis leading to hemorrhages in the extremities. The hepatic tissue was abnormal, being particularly poor in hematopoietic cells; blood cell macrocytosis was attributed to impaired erthropoiesis. Treatment of pregnant rabbits with folic acid plus vitamin B12 or with folinic acid was able simultaneously to prevent the fetal blood cell macrocytosis and the congenital abnormalities.