HUMAN erythrocytes contain two haemoglobins, a major component Hb A (α2β2) comprising 97 to 98% of the total Hb, which is actively synthesized in both nucleated red cells and reticulocytes and a minor component Hb A2 (α2δ2), which is produced chiefly in nucleated cells. This is because there is little or no δ chain synthesis by the reticulocyte stage of red cell maturation1,2. It has been shown recently that this early cessation of synthesis is not confined to the δ chain of Hb A2 but is observed also in the δβ fusion chain of Hb Lepore2–4. As the latter has the same N-terminal amino acid sequence as the δ chain of Hb A2 and presumably their mRNAs have the same 5′ nucleotide sequence, these observations suggest that the early decline of δ and δβ chain synthesis may somehow be connected with this region of mRNA. We have tested this possibility by examining the in vitro synthesis of the anti-Lepore haemoglobin, Hb Miyada, which has a non-α chain comprising the N-terminal residues of the β chain fused to the C-terminal residues of the δ chain5.