Some types of nondeletional heterocellular hereditary persistence of fetal hemoglobin (HPFH) appear to be caused by mutations in the β globin gene cluster near the γ globin genes, while in other cases the condition is associated with a gene or genes outside the β globin gene complex. We have used DNA probes for chromosome 11 markers to localize the HPFH determinant in a large Australian kindred with nondeletional heterocellular HPFH. In 13 of the 58 family members studied the Hb F levels are increased to between 1.8% and 7.9%, the Hb F being composed predominantly of Aγ chains. All family members were typed for restriction fragment length polymorphisms detected by probes from the β globin gene complex, and the nearby genetic markers D11S12, INS, and HRAS. Linkage analysis showed HPFH is closely linked to the β globin gene cluster (confidence limits of 0,0.0-0.19), D11S12 (0, 0.0-0.23) and the insulin gene (0,0.0-0.11). These data and the γ chain composition are consistent with HPFH in this family being caused by a mutation within the β globin gene cluster.