In patients with heterozygous β-thalassemia, the β/α synthetic ratio in marrow erythroid cells incubated in vitro is 1, whereas in reticulocytes the ratio is 0.5. These ratios reflect the equal synthesis of the two chains on the polyribosomes of the bone marrow and unequal synthesis on the polyribosomes of the peripheral blood reticulocytes. α- and β-chain synthesis is also equal in marrow cells in vivo. Equal synthesis is probably due both to a decrease in α-chain synthesis and an increase in β-chain synthesis in bone marrow erythroid cells and may contribute to the absence of overt hemolysis due to excess α-globin chain accumulation in heterozygous β-thalassemia.