We later showed that injected Rh-positive feetal cells can be cleared satisfactorily from the circulation of Rh-negative post-menopausal female volunteers. This was effected by the injection of 5 ml y-globulin with a very high titre of 'incomplete' anti-D, i.e. without saline activity (Clarke et al. 1963).As a result we felt that the immunization of Rh-negative women by pregnancy should be preventable in most cases if the usual antigenic stimulus was transplacental haemorrhage (TPH) and if the relevant TPH usually took place at or near delivery. We therefore investigated these points by examining, for feetal cells, the blood of Rh-negative primipare after delivery and testing their serum for antibodies three and six months later (Woodrow et al. 1965). We found that there is a significant relationship between the detection of feetal cells and subsequent anti-D production, and the greater the number of foetal cells found, the greater is the likelihood of immunization.Even so, we think that the majority of Rh immunizations by pregnancy are due to TPH of 'The following are taking part in this research work: