Serum alpha1 antitrypsin levels and phenotypes were examined in 1000 healthy adults in Northern Ireland. The Pi phenotype M accounted for 87-8% Pi MS for 7-3% and Pi MZ for 3-5%. The percentages of the rarer phenotypes were Pi FM 0-3, Pi IM 0-6, Pi SZ 0-4 and Pi Z--0-1. The range of alpha1 antitrypsin levels in different phenotypes showed considerable overlap. Pi MS, Pi MZ and Pi SZ sera gave mean levels of 81%, 61% and 40% respectively of the mean level obtained in Pi M sera and that of the Pi Z--minus serum, 5%. Two distinct types of anomalous pattern were encountered in sera from ten subjects.