To study the function of globin-chain genes, in vitro synthesis of globin was measured in reticulocytes concentrated from the peripheral blood of 7 subjects doubly heterozygous for an alpha-chain abnormality (Hb G-Philadelphia) and a beta-chain abnormality (Hb S or C). Each had a deficit of alpha-chain synthesis compatible with an alpha-thalassemia-like syndrome. The data are also compatible with the quantitative expression of variable reduplication of the alpha-chain locus in man.