DNA amplification combined with hybridization with 32P-labeled synthetic oligonucleotide probes has been used to identify base substitutions in the 5' promoter region of the A gamma globin gene in members of eleven families from China, Sardinia, Canada, and the United States who had a heterozygosity for the A gamma-beta+-hereditary persistence of fetal hemoglobin (HPFH), and in members of six black families with a possible G gamma-beta+-HPFH heterozygosity. All three known A gamma types were observed, ie, the British type (-198, T----C), the Chinese type (-196, C----T), and the Green type (-117, G----A); the latter has been found in a black family. Of the six families with G gamma-beta+-HPFH, three had C----G at -202 and none T----C -175. Conditions for hybridization of amplified DNA with the specific probes are provided and the usefulness of the technique is discussed. The increase in numbers of A gamma(G gamma)-beta+-HPFH heterozygotes with specific base substitutions greatly enhances the probability of a direct correlation between these substitutions and the increase in the production of a specific gamma chain.