We have tested the hypothesis that the protease inhibitor phenotypes MZ and MS are disadvantageous and reduce survival by comparing the prevalence of these phenotypes in a group of 707 very old people (hospital patients) with the prevalences reported in younger populations of blood donors. The MS and MZ phenotypes appear to be no less common among those who have survived to old age, but a highly significant difference was found in the occurrence of the M subtypes. The Ml type was more common in the elderly, and the M heterozygotes were less common than would be predicted from the reported incidence in younger groups and from the Hardy-Weinberg equilibrium. This discrepancy appeared to be smaller in subjects of Mediterranean origin than in those of British or Irish genetic background.