We have used a new method of DNA analysis for the rapid prenatal diagnosis of sickle cell anemia in two fetuses at risk for this disease. This method of detecting the sickle gene is a modification of standard restriction-enzyme techniques and requires only a small amount of DNA. The first step involves a 200,000-fold enzymatic amplification of the specific beta-globin DNA sequences that may carry the sickle mutation. This provides a sufficient quantity of DNA for the analysis. Next, a short radiolabeled synthetic DNA sequence homologous to normal beta A-globin gene sequences is hybridized to the amplified target sequences. The hybrid "duplexes" are then digested sequentially with two restriction endonucleases. The presence of beta A- or beta S-globin gene sequences in the amplified target DNA from the patient determines whether the beta A-hybridization probe anneals perfectly or with a single nucleotide mismatch. This difference affects the restriction-enzyme digestion of the DNA and the size of the resulting radiolabeled digestion products, which can be distinguished by electrophoresis followed by autoradiography. This method is sufficiently sensitive and rapid that the prenatal diagnosis of sickle cell anemia can be made on the same day that the fetal DNA is made available. It can also be applied to the diagnosis of hemoglobin C disease.