Restriction endonuclease mapping of nondeletion alpha-thalassemia determinants from a variety of racial groups showed no detectable abnormalities within a 40-kilobase region of the zeta-alpha globin gene cluster. By using a zeta-specific probe, we defined three different types of interactions that give rise to Hb H disease, each involving a nondeletion alpha-thalassemia haplotype. mRNA analysis showed further diversity within these groups, indicating that there are at least three nondeletion determinations.