RECENTLY, Finn et al. have suggested that it may be possible to prevent the antigenic stimulation of Rh-positive fœtal cells in Kh-negative mothers after the cells cross the placental barrier. Their reports confirmed the theory that intact fœtal red cells gain access to the maternal circulation and induce antibody formation and that higher probabilities of Rh iso-immunization occur when significant transplacental haemorrhage has taken place. Their proposal for prophylaxis and the experimental work reported are based on the recognition that ABO incompatibility confers protection against Rh immunization. They reported that the administration of Rh-positive (D) cells to volunteer recipients, followed shortly thereafter by anti-D, results in the rapid disappearance of these cells from the circulation and prevents Rh sensitization. Although the results show that this may obtain in their experiments, we feel that there are certain dangers inherent in their technique which could result in more rapid iso-immunization or a higher probability of iso-immunization.