alpha 1-Antichymotrypsin (ACT), a component of the senile plaque of the Alzheimer's disease (AD) brain, has a possible role as a molecular chaperone in developing AD pathology. This study was a search for the possible association of the two structural polymorphisms of ACT, Ala15-->Thr and Met389-->Val in the Japanese population. In 101 AD patients, genotype and allele frequencies of the two polymorphisms did not differ from those of 104 age-matched healthy controls. However, in those subjects in which the apolipoprotein epsilon 4 allele was absent, the frequency of the Ala15 homozygote was significantly higher in the AD patients than in controls. This suggests that the Ala15 homozygote state may be a susceptibility marker for AD, interacting with apolipoprotein E genotype.