The concept of erythroblastosis fetalis as a hemolytic familial disease entity has become firmly established. Ottenberg1 was the first to suggest an antigen-antibody relationship between the red blood cells of the fetus and the blood of the mother. Macklin2 observed that mothers of infants with erythroblastosis fetalis were subject to spontaneous abortions, miscarriages and stillbirths. Landsteiner and Wiener3 demonstrated the Rh factor, an agglutinogen transmitted as a simple mendelian dominant characteristic and found in the red blood cells of 85 per cent of human beings. An erythroblastotic fetus inherits from the father the agglutinogen which determines the Rh positivity of the blood. Levine, Katzin and Burnham4 and Levine, Burnham, Katzin, and Vogel5 demonstrated the association of erythroblastosis fetalis with the Rh factor on the basis of isoimmunization. The modern concept is actually an application of the principles of isoimmunization laid down by Ehrlich and Morgenroth