Using isoelectric focusing (IEF) in thin-layer polyacrylamide gel, the polymorphism of the serum alpha-1-antitrypsin (Pi system) was investigated in 335 healthy unrelated Japanese individuals living in Miyagi prefecture, the northern part of Japan. Six common and five rare variant phenotypes were identified in our population samples, and the estimated allele frequencies for the genes PiM1, PiM2 and PiM3 were 0.718, 0.238 and 0.044, respectively. Family studies (n = 46) showed an autosomal codominant inheritance, and no exclusion was found in 23 mother-child pairs.