Hemoglobin synthesis in a 34-year-old man of Laotian ancestry with Hb E/β°- thalassemia was studied. Hemoglobin electrophoresis exhibited only Hb F and Hb E. Reticulocytes and bone marrow cells showed no Hb A synthesis. The proportion of Hb F increased (from 14 to 50%) with the density of RBCs fractionated on a discontinuous gradient of Stractan. Reticulocytes were also heterogeneous with regard to their density and their Hb F synthesis: the proportion of Hb F synthesis increased with their density. Fractionation of bone marrow cells by density on a discontinuous gradient of Stractan provided populations of cells at different stages of maturation, the more mature having the greater density. Study of hemoglobin synthesis in bone marrow cells indicated that the proportion of γ-chains synthesized increased and the pool of free α-chains decreased with erythroblast maturation; in contrast, the proportion of β E-chains compared to α-chain synthesis remained constantly low. These results provide evidence that the increased amount of Hb F found in the blood of the patient was mostly due to the positive selection, in bone marrow and in peripheral blood, of cells containing Hb F. In contrast, colonies of early erythroid precursors derived from the blood of the patient exhibited an enormous reactivation of Hb F synthesis in culture. The low proportion of β E-chain synthesis (about 30% of α-chain synthesis) remained unchanged during the erythroid maturation of most immature erythroblasts to reticulocytes. The combination of two thalassemic alleles, one totally deficient (β°) the other partially deficient (βE), can explain the clinical severity of the association β⁰-thalassemia/Hb E.