The disappearance kinetics of a standard dose of 300 micrograms of Rh immune globulin given at 28 weeks' gestation was investigated. Ten Rh-negative unsensitized women were given 300 micrograms of Rh immune globulin at 28 weeks and blood samples were drawn at weekly intervals until delivery. Sera were titrated with Ror red blood cells in a saline solution-antiglobulin method. Titers ranged from 1 to 4 by the first week after injection. In nine of 10 patients the titers were zero at 49 to 70 days after injection. With R2R2 red blood cells and LISS, papain, polybrene, or a combination of methods, Rh immune globulin could still be detected until delivery in four of the nine patients. Because these serologic methods can detect 5 ng/ml anti-D, five of 10 subjects may not have been protected for 8 to 29 days before delivery. This study indicates that a titer of less than or equal to 4 is expected for passively acquired anti-D during pregnancy, and that antepartum administration of Rh immune globulin may not always produce detectable passive antibody for as long as theoretically predicted.