In 1912, Rautmann1 first applied the term Erythroblastose to a condition in a hydropic fetus in which there was evidence of excess hemopoiesis in the liver, spleen and kidney. Since that time the term has been used to designate a group of conditions including fetal hydrops, fatal jaundice of the newborn and congenital anemia. Since the discovery of the Rh factor by Landsteiner and Wiener2 and the application of the knowledge gained through this discovery to the problem of erythroblastosis by Levine and his associates,3 the term has been employed to designate the condition arising through incompatibility of blood factors between mother and fetus. Confusion now exists in the literature as to what constitutes erythroblastosis. There are those who state that hemolysis is the distinguishing feature of the condition; others state that the essential criterion is the presence in the fetus, at or near term, of extramedullary