Globin biosynthesis was studied in both erythroid precursors and reticulocytes of three individuals with heterozygous alpha-thalassaemia. In contrast to the finding of equal or nearly equal alpha and beta chain synthesis in the marrow of patients with heterozygous beta-thalassaemia previously examined, our studies showed equal degrees of unbalanced globin synthesis in both reticulocytes and nucleated-erythroid cells of alpha-thalassaemia heterozygotes. Greater stability and less susceptibility to proteolysis of the excess beta-chain formed in alpha-thalassaemia may explain our findings.