SUMMARY.Routine antenatal Rh immunopro‐phylaxis would substantially increase the use of anti‐D Ig in the U.K. As availability of anti‐D Ig is one factor influencing a decision to introduce routine antenatal prophylaxis, a trial was undertaken to test the efficacy of a lower dose of anti‐D Ig than that used in earlier studies. RhD‐negative primigravidae were randomized as controls or recipients of two doses of 250 iu of anti‐D Ig given at 28 and 34 weeks gestation. Blood samples were tested at delivery and at 6 months postpartum for the presence of immune anti‐D, and again later if results were equivocal. Nine (1.5%) out of 595 control patients had immune anti‐D at follow‐up at 6 months and later; 4 (0.78%) of 513 treated women were immunized. It was concluded that, while two doses of 250 iu of anti‐D Ig may reduce allo‐immunization, they are not as effective as two doses of 500 i