A serum containing anti-Rh+ antibodies given to women at high risk for having a baby with erythroblastosis fetalis (i. E. , Rh- women with Rh+ mates). This must be done for the first and all subsequent pregnancies. The injected antibodies quickly agglutinate any fetal red cells as they enter the mother's blood, thereby preventing her from forming her own antibodies. Since the serum given the mother is a passive form of immunization, it will shortly leave her blood stream.