Three→β-thalassemia homozygotes were found in a Mexican family. Both parents and two siblings had heterozygous 63-thalassemia with about 10% Hb F, mild microcytosis and mild hypochromia, while three siblings were normal. Hb F, which was the only Hb component in the homozygotes, had equal quantities of Gγand Aγchains as in GγAγ-→β-thalassemia. The homozygotes had comparable erythrocytic indices which were about the same as those of the heterozygotes. However, two were clinically and hematologically healthy but the third had a severe chronic hemolytic anemia and a more severein vitrochain synthesis imbalance than her homozygous sisters. Comparison of these cases with other GγAγ63-thalassemia homozygotes and with GγAγHPFH homozygotes indicates the possibility that the proliferation of F-cell precursors may be defective in→β-thalassemia.