Haemoglobin synthesis was studied in bone marrow erythroblasts and in reticulocytes of 4 children with beta O-thalassaemia major and of 7 children with beta +-thalassaemia major. In patients with beta O-thalassaemia the gamma/a ratio was found to be lower in bone marrow than in peripheral blood. On the contrary, in patients with beta + thalassaemia the beta + gamma/a ratio was more balanced in bone marrow, where the beta-chain synthesis was higher, than in reticulocytes. This last result could be explained by the presence of an abnormal m-RNA for beta-chains in beta +-thalassaemia.