Previous studies of β-thalassemic reticulocytes have implied a decreased amount of functional β-mRNA but unimpaired translation of the β-mRNA present. However, the β/α synthetic ratios in β-thalassemic marrow are higher than those observed in reticulocytes of the same patients. This could imply that marrow cells contain an abnormally functioning β-mRNA no longer active in reticulocytes. To test the function of mRNA found in marrow, intact cells were incubated with [³⁵S]methionine and the relative amounts of nascent α- and β-chains on polysomes of different sizes were measured by tryptic digestion and determination of the specific activities of the respective peptides. Results showed that in normal and β-thalassemic marrow, as well as in reticulocytes, β-chain production, though deficient, occurs predominantly on larger polysomes than the production of α-chains. In one patient with severe thalassemia and very little production of β-chains in marrow or reticulocytes, δ-chain synthesis was found predominantly on larger polysomes than α-chain synthesis. These results indicate that in β-thalassemic as well as in nonthalassemic marrow and reticulocytes, each β- and δ-mRNA initiates protein synthesis at a rate faster than does each α-mRNA, and suggest that the β-mRNA in contact with polyribosomes is normally functioning but quantitatively deficient in β-thalassemic marrow as well as in reticulocytes. No translational defect was detected in a similar study performed in reticulocytes of a patient with hemoglobin H disease, suggesting a normally functioning mRNA in contact with polyribosomes in this condition as well. In both thalassemias, unbalanced synthesis of α- and β-chains was more pronounced on polysomes than in completed chains. This difference possibly reflects a compensatory delay in translation of the nonthalassemic chain, which is present in excess.