High fetal haemoglobin levels of 5-15% are present in adult heterozygotes for delta beta-thalassaemia as the result of large deletions of DNA. We have cloned DNA spanning the deletion breakpoint for a new Indian delta beta-thalassaemia associated with mild anaemia. The 5' breakpoint is at 42151 of GenBank file HUMHBB, which is about 1 kb 3' of the A gamma globin gene poly A site at 41003. On the 3' side of the breakpoint, the sequence is homologous to L1 (KpnI) repetitive DNA located 3.6-10 kb 3' of the beta-globin gene: Indian delta beta-thalassaemia DNA is 74% homologous to the inverted complement of HUMHBB from 69849 to 70020, followed by a region 78% homologous to the direct sequence of HUMHBB from 70534 to 71010. The precise location of the 3' endpoint of this deletion has not been determined, but it is within L1 sequences located more than 10 kb 3' of the beta-globin gene.