A population survey carried out in southern sardinia on more than 5000 people has shown that delta beta (F)-thalassaemia, with a gene frequency of 0-00088, is a rare trait in this population. We examined the members of three families segregating for both delta beta- and beta(0)-thalassemia and a number of delta beta carriers identified during the screening. The doubly heterozygous children suffer from a mild form of Cooley's disease with non-alpha/alpha biosynthetic ratios within the range of values observed in beta (0)-thalassaemia homozygotes. Three of them have been transfusion dependent for some time. The delta beta carriers, although in many respects showing the usual picture of delta beta-thalassaemia, such as abnormal red cell indices, normal Hb A2, Hb F heterogeneously distributed in the erythrocytes, and low beta/alpha synthetic ratios, have unusually high levels of Hb F (range 10 to 20%) and particularly low glycine content (range 0.02 to 0.14 residues) in the isolated gamma CB3 peptide. These results have led us to the conclusion that the delta beta-thalassaemia found in Sardinia is different from the similar kind of delta beta defect found in Negroes and in other Mediterranean populations, including continental Italians.