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aborting. Hence the incidence of post-abortion sensitisation probably cannot be eliminated. Nevertheless, much can be done to reduce the number of cases appreciably. Surprisingly, many doctors are still unaware that anti-D immunoglobulin should be administered in these cases. Occasionally it is difficult to pinpoint the time of the abortion. Samson and Mollison,5 however, found that administering anti-D immunoglobulin up to 13 days after an injection of Rh-positive cells was effective in preventing sensitisation. Anti-D immunoglobulin should be given even at this late stage after a possible abortion. Roughly 20 cases a year of post-abortion or administrative failures of protection occur in the Yorkshire region-that is, a third of all new cases. If this region is typical eliminating these cases would greatly reduce the total incidence in Great Britain.Primigravidae who are sensitised before anti-D immunoglobulin is normally administered make up most cases of socalled failures of protection. Several centres have instituted a programnme of antenatal administration of anti-D immunoglobulin, which is also being actively considered in Great Britain. Regimens vary from one dose, usually at 28 weeks, as in the Hamilton (Canada) series and a Swedish series, to a twodose regimen usually at 28 and 34 weeks in an Australian programme and the Manitoba project (unpublished data). Preliminary results suggest that antenatal administration of anti-D immunoglobulin reduces the incidence of antibodies detected at six months after delivery, but it is too early to be certain that it reduces the number of sensitised mothers in subsequent pregnancies. If it is shown that antenatal injection does reduce the incidence of sensitisation the incidence in the Yorkshire region would decrease by 20 to 30 cases a year. If this again is typical it would mean a decrease of 300-400 cases a year in Great Britain. Nevertheless, it would entail administering three times as much anti-D immunoglobulin as at present, a costly enterprise. Moreover, the number of new cases might be similarly reduced by administering anti-D immunoglobulin after all Rh-negative deliveries and after every abortion.One of our findings is of theoretical as well as practical interest. The fall in the incidence of Rh antibodies in Rhnegative women was 700/ ; the fall in the incidence of Rhpositive mothers with antibodies was 19O0, which presumably reflects the fall in the number of women having babies. Nevertheless, the fall in the number of Rh-negative women with nonRh antibodies was 3500. This suggests that anti-D immunoglobulin may have a non-specific effect, and supports the thesis of Woodrow and others in Liverpool6 that the mechanism of anti-D immunoglobulin protection is not antigen-specific. More data on this topic are needed.We thank all obstetricians in the region for their help and cooperation, and for willingly making available the notes ofthe patients concerned; and the clerical staff for patiently obtaining the information required.

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