We have used restriction endonuclease mapping to study a deletion involving the beta-globin gene cluster in a Mexican-American family with gamma delta beta-thalassemia. Analysis of DNA polymorphisms demonstrated deletion of the beta-globin gene from the affected chromosome. Using a DNA fragment that maps greater than 40 kilobases (kb) 5′ to the epsilon-gene as a probe, reduced amounts of normal fragments were found in the DNA of affected family members. Similar analysis using radiolabeled DNA fragments located 3′ to the beta-globin cluster has shown that the deletion extends more than 17 kb 3′ to the beta-gene, but terminates before the 3′ endpoint of the Ghanian HPFH deletion. Hence, this gamma delta beta-thalassemia deletion eliminates over 105 kb of DNA and is the first report of a deletion of the entire beta-globin gene cluster.