Alpha-1-antitrypsin (Pi) phenotypes have been determined in 40 patients suffering from Down's syndrome. Thirty-six of the patients were found to have a normal M phenotype, whereas two deficient phenotypes of the MS variety were observed. In addition, two M variants were noted. The significance of an M variant phenotype in some patients with Down's syndrome is not completely understood and is currently under investigation. Since the majority of the patients had a normal alpha-1-antitrypsin phenotype, the results of this study indicate that a deficiency in alpha-1-antitrypsin plays no role in the respiratory fragility of individuals with Down's syndrome.