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The author establishes that there is no reason to think that the isoimmunization must be limited to the Rh factor, especially in pregnancy cases, because such a condition brings the ideal means for its presence. A brief historical review is presented, establishing some evidence for the antigenic capacity of the A and B blood factors, such as the experimental production of serum in animals, the increase of agglutinin titre following an incompatible transfusion or intramuscular blood injection and heterospecific pregnancies. Some fetal antigens are analyzed, as well as the possible pathogenic process of sensitization. Some factors of the antigen-antibody conflict in the fetal organism are studied. The pregnancies are also analyzed from the viewpoint of the fetal maternal incompatibility explaining the low frequency of the observed cases, due to nonpermeable placentas, the anergy of the mother, and the fetal characteristic of being secretor or nonsecretor, considering that the clinical form of the fetal alteration depends upon the presence or absence of A and B substances in fetal tissues. The writer gives as a basis to consider a true isoimmunization the presence in the blood stream of erythroblasts with nuclear alterations, and a positive "developing test." A clinical case of twins in which the sick twin was of incompatible blood type in respect to the mother is presented.

The experience of the William Buchanan Blood, Plasma and Serum Center with routine Rh typing since May 1944 has led to the conviction that this additional routine service is essential for transfusions of maximum safety.1 It also appears that infant mortality due to hemolytic disease of the newborn (erythroblastosis fetalis) can be decreased through the additional information acquired from routine Rh typing of all obstetric cases and newborn infants; also routine Rh typing should be available to military services, first, because in the great number of whole blood transfusions now being given there is increased opportunity for Rh negative individuals to be sensitized so that later transfusions may be dangerous and, second, the necessity for multiple transfusions following severe battle injuries increases the opportunities for isoimmunization with the Rh antigen and resultant transfusion accidents.The chief problem in routine Rh typing is to secure a sufficient and continuing source of potent