This study shows a marked and protracted activation of HbF synthesis in homozygous β°‐thalassaemia patients transplanted from HLA identical siblings heterozygous for β‐thalassaemia, as compared to patients transplanted from normal donors. HbF synthesis in recipients was much higher in relation to the corresponding bone marrow donor values either normal or heterozygous for β thalassaemia.

γ‐chain synthesis and G_γ/A_γ ratio were also studied in peripheral blood BFU‐E from recipients and their donors. BFU‐E from donors heterozygous for β‐thalassaemia showed higher γ chain synthesis as compared to normal donors. Peripheral blood BFU‐E γ/β+γ ratios and G_γ percentage were higher in recipients than in their corresponding donors both normal or heterozygotes.

The marked and protracted reactivation of HbF synthesis in recipients of heterozygous β‐thalassaemia bone marrow most likely results from an increased erythropoietic stress on erythroid progenitors. In order to obtain adequate Hb levels heterozygous β‐thalassaemia bone marrow should produce more red blood cells to compensate for the low MCH. The magnitude of activation of HbF synthesis was very variable. This variability may result from inherited differences in the capacity of reactivation of HbF synthesis of red cell progenitors from heterozygous β‐thalassaemia under stressed erythropoiesis.