Globin chain synthesis analysis was carried out in 14 obligate beta 0-thalassemia heterozygotes, i.e., parents of children with thalassemia major or intermedia due to homozygous beta 0-thalassemia, with almost normal red blood cell indices and isolated increase of Hb A2 levels. alpha/beta globin chain synthesis ratios were almost balanced or slightly reduced (0.62-1.30). This finding suggests that these heterozygotes carry both alpha- and beta-thalassemia genes. However, alpha-globin structural gene mapping by restriction enzyme analysis must be carried out to confirm this explanation.