ON THE BASIS of a large unselected case material, an attempt was made in the preceding papers * to draw the picture of ABO heterospecific pregnancy as it is ordinarily seen and to describe the effects on the isoantibody patterns of mother and child, as well as on the clinical and hematologic condition of the newborn infant. Against this background, the present study attempts similarly to characterize the pathologic variants of ABO heterospecific pregnancy and to offer observations which may aid in an understanding of the etiology and pathogenesis of ABO hemolytic disease. The case material presented for this purpose, in contrast to that of the earlier studies, was selected in the sense that it consisted only of infants suspected for some reason of having hemolytic disease of the newborn due to ABO incompatibility. All cases of hemolytic disease of the newborn due to maternal isoimmunization against Rh or rare antigens