Thirty-one Caucasian families characterized by having one or more infants with ABO hemolytic disease of the newborn were ascertained and investigated. All the mothers were blood type O. Of a total of 47 affected type A infants, 2 were subtype A2 and the rest A1. Thirty-three of the affected infants were female and 14 male, revealing an obvious effect of sex. No association was found between secretor status and ABO hemolytic disease. Within subtype A1 the relative strength of the paternal antigen was not significantly different from that of a control sample of males. Some aspects of the complex etiology of ABO hemolytic disease have been discussed.

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