In a previous study, molecular cloning of the alpha-globin genes from a patient with nondeletion Hb-H disease (genotype--/alpha alpha) showed that a single nucleotide mutation (CTG to CCG) in one of the genes resulted in a leucine to proline substitution. This paper describes the approach we used to detect the abnormal alpha-globin chain. The chain was identified using a cell-free translation system. It turned over rapidly both in vitro and in vivo in the patient's reticulocytes. The unusual feature of this unstable alpha-globin is that the alpha-globin deficiency causes alpha-thalassemia. Simple heterozygotes for this lesion (alpha Pro alpha/alpha alpha) resemble alpha-thalassemia carriers and do not exhibit the hemolytic anemia usually associated with unstable hemoglobins.