The serum of an Rh0 (D)+ Caucasian female with a prior history of transfusions contained anti- LWa , reacting moderately with Rh0 (D)+ and weakly with Rh0 (D)- red cells at the antiglobulin phase. Since transfusions were required, Chromium (51Cr) survival studies and mononuclear phagocyte assays (MPA) were used to predict in vivo survival. The MPA value of 6% of the positive control predicted a low likelihood of clinically significant extravascular destruction and 51Cr survival studies indicated greater than 95% survival at 1 h and 78% survival at 20 h for rr erythrocytes. Transfusion of 4 units of Rh0 (D)-, serologically incompatible red cells increased the hemoglobin from 5.2 to 11.4 g/dl and the titer of anti- LWa against rr red cells from 4 to 1,024 7 days after the transfusion. A second 51Cr survival also indicated normal survival of rr erythrocytes. 3 additional Rh0 (D)- units were successfully transfused.