Thirty-two patients who underwent bone marrow transplantation for hematologic malignancies were studied for hemoglobin-F as an indicator for fetal erythropoiesis. Two different patterns of response were noted. One group of patients who had an elevated HbF level prior to marrow grafting later showed a marked reactivation of HbF synthesis, whereas the other group of patients who had normal HbF levels prior to transplantation failed to exhibit such an increase. This phenomenon occurred independently of ages of marrow graft donors or recipients, the type of underlying hematologic malignancy, or remission induction therapy prior to preparation for transplantation, pretransplant hemoglobin levels, transfusion with red blood cells, red cell volume, and production of reticulocytes.