The Rh Laboratory could find no evidence that Rh immune globulin (RhIG) reduced the incidence of progression of Rh immunization in a clinical study of 36 weakly Rh-immunized women given 300 micrograms of RhIG at 6-week intervals during pregnancy and after delivery, and in a retrospective study of the natural course of weak Rh immunization in pregnant Rh-negative women not given RhIG and others given RhIG at 28 weeks' gestation and/or after delivery of an Rh-positive baby. Although the failure of about one-third of weakly immunized Rh pregnant women to show any progression of their Rh immunization may be due to relative immune unresponsiveness in some, we believe that most do not have any increase in their Rh immunization because they have no further exposure to Rh-positive fetal red cells. Serial Kleihauer testing of pregnant women at 2-week intervals is being carried out to test this hypothesis.