We have studied the hematology, hemoglobin composition, and globin gene arrangements in one young Turkish boy with a beta zero-thalassemia homozygosity and in 11 of his relatives. Evidence is presented that the chromosome with the beta zero-thalassemia determinant carries a gamma-globin gene quadruplication, perhaps in a -G gamma-G gamma-G gamma-A gamma-gene arrangement. The eight gamma-globin genes in this patient produced G gamma and A gamma chains in a 95 to 5 ratio, and nearly 99% of the patient's hemoglobin was of the fetal type. The clinical condition resembled that of a thalassemia intermedia. HbF levels in eight beta-thalassemia heterozygotes varied between 0.5 and 4.2% and the percentages of G gamma in this HbF averaged at 87% or 95%; this level is to some extent related to the haplotype of the normal chromosome. All subjects carried four alpha-globin genes; a new BglII polymorphism was observed within the psi alpha-globin gene.