A stable polymorphism of the Pi alleles M1, M2 and M3 close to their observed frequencies has been predicted, simply by relating fitness to the proportion of each genotype having α1-antitrypsin (α1-AT) levels greater than 100 mg/100 ml. By further specifying that the rare S and Z genotypes with low α1-AT levels have enhanced fertility, equilibria of the M, S and Z alleles have also been predicted close to their observed frequencies; however, none of these latter equilibria is stable, mainly because the predicted fitness of the SS genotype is too great.