Abstract. Rh-negative women, immunized to Rh by previous pregnancies, with only low concentrations of IgG anti-Rh(D) in their plasma were assigned at random to test and control groups (7 subjects in each group). Both groups were challenged with an intravenous injection of 0.28 ml of Rh-positive red cells; in addition, the test group received 500 μg anti-Rh intramuscularly. 2 weeks after the injections, all subjects showed an increase in plasma anti-Rh concentration; levels in test and control groups were similar. It is concluded that in Rh-immunized subjects with low levels of IgG anti-Rh a secondary response to Rh cannot be prevented by giving passively administered anti-Rh with the red cells.