Female rabbits became immunized during pregnancy to the rabbit blood group factors G or g in five out of ten instances in which the red cells of the fetuses carried one of the factors absent in the mother. Antibodies so produced were of low titer and disappeared in all cases within 6 weeks after the birth of the litter. Repeated pregnancies did not result in additive increases in titer.

Antibodies to the G-g factors, whether produced by the injection of red cells or by pregnancy, crossed the placenta readily from mother to fetus and were found at birth (and prior to nursing) associated with the red cells and in the serum of the fetuses. The rabbit placenta appeared to be equally permeable to the agglutinating and coating antibodies.

The implications of these findings and their relation to the pathogenesis of erythroblastosis fetalis are briefly touched upon.

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