DNA from members of 10 Black families with conditions considered to be G gamma (delta beta)0-thalassaemia or G gamma (delta beta)0-HPFH were studied by using restriction enzyme analysis. One or more affected members from each family were shown to have the same deletion of 34 kbp of DNA in the human beta-globin gene cluster. A clone spanning the deletion was isolated from the DNA of one such person and studied in detail. The deletion removed part of the A gamma and all of the psi beta, delta and beta-globin genes and is different from the four previously identified deletions which caused a condition presently known as (A gamma delta beta)0-thalassaemia.