In two Chinese patients with homozygous beta(0)-thalassemia, messenger RNAs from peripheral blood reticulocytes and the bone marrow failed to direct beta-chain synthesis in vivo and in vitro in a cell-free system. Molecular hybridization showed that the beta cDNA annealed to the RNAs at almost the same rate as the alpha and gamma cDNA. The beta cDNA-RNA hydrid formed efficiently and was thermally stable, whereas hybrids between gamma and beta sequences formed slowly and denatured at a significantly lower temperature. Thus, we conclude that the beta cDNA was annealing to beta-globin sequences in these two patients, and that nonfunctional beta-globin mRNA was present. Similar results were obtained in the reticulocyte RNA from an Italian patient with homozygous beta(0)-thalassemia.