The prevalence of the BamH I site 3' to the beta globin gene in Chinese people was determined in 123 normal subjects, 40 patients with heterozygous beta thalassaemia, and 25 patients with homozygous beta thalassaemia. The site was present in 71.1% and absent in 28.9% of the chromosomes carrying normal beta genes. All 25 patients with beta thalassaemia major had the site. This BamH I polymorphism may be used for prenatal diagnosis in about 29% of the pregnancies at risk.