As immune globulin anti-D given in the immediate post partum period fails to prevent the development of anti-D antibodies in about 1.5-2% of women at risk, probably as a result of feto-maternal bleeding during pregnancy, 300 micrograms of immune globulin anti-D was administered to 609 Rh-negative women in the 28th gestational week. Three hundred and forty-six had Rh-positive babies, and were given additionally 200 micrograms anti-D post partum. Of these, 291 had an antibody screen test done 10 months after delivery. No anti-D antibodies were found. The test results of the study group were compared with those of a control group of 354 women who did not receive prenatal immune globulin, but otherwise the same examinations and treatment. In this group, 322 had a screen test performed 10 months post partum or in their next pregnancy, when 1.8% had anti-D antibodies. The difference in immunization incidence between the groups was significant (p less than 0.05). There was no difference between the groups regarding the number of women with fetal erythrocytes in serum after delivery, but the number of fetal erythrocytes found was significantly lower in the group receiving prenatal prophylaxis (p less than 0.001). No adverse effects were found in the infants exposed to anti-D prenatally.