Deficiency of alpha 1-antitrypsin can be diagnosed by DNA methods that are quick and also give reproducible results. Direct DNA analysis of the M, S, and Z alleles of the alpha 1-antitrypsin gene is possible when two molecular biology techniques are combined: a part of the gene is enzymatically replicated according to the pattern of the human DNA that is to be investigated. On hybridization with the various allele-specific pieces of synthetic DNA the copies that have come about in this way give a signal corresponding to the genotype. Only a small amount of material (0.5 microgram DNA) is needed for the investigation. This method is thus suitable for prenatal diagnosis when this is requested by members of a family known to be affected. A total of 53 samples were typed and investigations were carried out in six families.