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A series of Rh-negative primiparae has been studied in order to gain further insight into the process of immunization by pregnancy. The distribution of foetal cell counts in blood samples taken after delivery was determined for 2,029 mothers giving birth to ABO-compatible babies and for 417 mothers with ABO-incompatible babies.A total of 760 mothers were tested for the development of Rh antibodies six months after the delivery of an ABO-compatible Rh-positive baby and 236 were further followed up through a second Rh-positive pregnancy. The incidence of anti-D six months after delivery is estimated to be 8.5%, and there is evidence of a direct relation between the count of foetal cells after delivery and the risk of developing antibodies. A further 8.5% of mothers were estimated to develop anti-D by the end of the second pregnancy, and it is postulated that these individuals had been primed by the first pregnancy. There is some evidence that the larger stimuli of Rh-positive blood in the first pregnancy are more likely to result in overt antibody formation, while the smaller stimuli are more likely to prime, antibodies not being detected until a second stimulus occurs during the second pregnancy.These findings are relevant to the programme for preventing Rh-immunization by injecting anti-D gammaglobulin.