In this study we have characterized by DNA analysis the molecular basis of an α -thalassemia condition found in an infant, with 16% Hb Bart's at birth, who developed an hematologic picture similar to the α -thalassemia carrier state. Restriction endonuclease analysis and hybridization with α and ζ specific probes have provided strong evidence that this patient carries a genetic compound of deletion a-thalassemia-2 lesion (-α ) and a non-deletion defect {(αα)th} with both a-structural genes intact on chromosome 16. He inherited the deletion a-thalassemia α-2 chromosome (-α) from the father and the chromosome with nondeletion α-thalassemia defect from the mother. Because the deletion of one, two, or three α-globin structural genes is associated with 1-2%, 5-6%, or 25%, Hb Bart's respectively, these findings suggest that the non-deletion chromosome {(αα )th'} contains two α -globin structural genes that are less active than a single a gene (-α).