A D-negative primigravida inadvertently received 450 ml of D-positive blood, after delivering her first D-incompatible infant. After 9,575 μg, of intramuscular IgG Rh immunoglobulin, both fetal and transfused erythrocytes exhibited immune clearance (4-5 days). No clinical reaction occurred but there was transient hypohaptoglobinemia and bilirubinemia from demise of the transfused red cells. The D-positive cell population exhibited a positive direct antiglobulin test at 48 h and for 3 additional days after intramuscular Rh antibody therapy. Moderate titers of the passively acquired Rh immunoglobulin were detectable for 165 days. No active Rh immunization developed in the recipient during 1 year of posttransfusion observation. 22 months later, during a second childbirth and challenge with another Dincompatible infant, again no active maternal Rh immunization developed. The preventive dose was 49 μg of Rh immunoglobulin/ml of D-positive transfused erythrocytes. This dose is equivalent to 20 μg of IgG anti-D/ml of whole blood.