The polymorphism of a restriction endonuclease site has been used for antenatal diagnosis of sickle-cell disease. In a normal person, the beta-globin gene was contained in a Hpa I-digested D.N.A. fragment 7.6 kilobases (kb) in length. In a family where the sickle gene was contained in a variant 13.0 kb fragment, restriction endonuclease mapping was used for antenatal diagnosis. The D.N.A. from amniotic-fluid cells produced both the 7.6 and the 13.0 bk beta-globin gene fragments, indicating the diagnosis of sickle-cell trait. This confirmed the diagnosis reached after investigation of a 100% sample of fetal blood. The method is sensitive and can be performed with cells obtained from 15 ml of uncultured amniotic fluid. This approach may prove useful in antenatal diagnosis of other genetic disorders.