alpha 1-Antitrypsin phenotypes and alpha 2-macroglobulin concentrations were determined for an Australian population of 286 Down's syndrome patients, 296 newborn babies, and 193 patients with non-specific mental retardation. There was no significant difference between the groups in the distribution of the phenotypes or level of heterozygosity. The phenotypic frequencies were 89.5% PiMM, 8.0% PiMS and 1.0% PiMZ among controls. The mean alpha 2-macroglobulin concentrations were investigated and it was shown that PiMZ phenotypes had a higher concentration, 2.67 +/- 0.27 g/l (newborns) and 2.74 +/- 0.32 g/l (Down's syndrome), in comparison with PiMS, PiSS, and PiMM.