Abstract. One third of very weakly Rh-immunized women show no increase in their level of Rh immunization during pregnancy and after delivery. Administration of 300 μg of Rh immune globulin at 6-week intervals does not alter the incidence of lack of progression of Rh immunization in such women. Thirty-three Rh-positive women who ultimately delivered ABO-compatible babies, had Kleihauer fetal transplacental hemorrhage (TPH) screening tests carried out at 2-week intervals during pregnancy and shortly after delivery. One third of the 33 women had no detectable TPH (8) or a TPH of 0.01 ml of fetal red cells (3). We conclude that one third of weakly Rh-immunized women failed to show any progression of their Rh immune response during pregnancy or after delivery because they were exposed to too few fetal red cells (0.01 ml or less) to produce such a response.