Twenty-six DNA samples from individuals either heterozygous or homozygous for beta thalassemia were analyzed by restriction endonuclease digestion, agarose gel electrophoresis, and Southern blot analysis to define DNA fragments containing portions or all of the beta globin gene. A total of twenty-seven genes affected by a beta thalassemia mutation and twenty-seven genes affected by a beta thalassemia mutation and twenty-two normal beta globin genes were examined in Italian, Greek, or Asian individuals. With all four restriction endonucleases used, the fragments generated from DNA of thalassemic individuals were identical to those found in DNA from normal. Thus, gross rearrangement or deletion within the genomic region containing the beta globin gene is not characteristic of mutations which cause a thalassemia. A third patient homozygous for pancellular hereditary persistence of fetal hemoglobin was shown to have complete deletion of the delta and beta globin genes.