Analysis of the physical arrangement of globin genes in cellular DNA is now feasible. High molecular weight DNA is digested with site-specific restriction endonucleases, fractionated in agarose gels, and hybridized with radioactive complementary globin cDNA either in situ or after transfer to nitrocellulose filters. This technique permits direct, autoradiographic visualization of DNA fragments containing globin gene sequences in DNA isolated from normal individuals and those with hemoglobinopathies. Comparison of the hybridization patterns of different DNAs provides a convenient, sensitive system for the detection of globin gene deletions or rearrangements. Study of homozygous α-thalassemia (hydrops fetalis) DNA revealed complete absence of specific DNA fragments containing a globin sequences. The sensitivity and clarity of the visualization of this deletion indicate that this new approach will be especially useful in the prenatal detection of globin gene deletions or rearrangements in amniotic cell DNA of fetuses at risk for severe thalassemia syndromes.