The concept of erythroblastosis fetalis as a hemolytic familial disease entity has become firmly established. Ottenberg 1 was the first to suggest an antigen-antibody relationship between the red blood cells of the fetus and the blood of the mother. Macklin 2 observed that mothers of infants with erythroblastosis fetalis were subject to spontaneous abortions, miscarriages and stillbirths. Landsteiner and Wiener 3 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 Burnham 4 and Levine, Burnham, Katzin, and Vogel 5 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