The phenotype distribution and gene frequencies of serum alpha1-antitrypsin in 856 healthy blood donors in Tokyo were examined by gel slab isoelectric focusing (pH 4--6). The allele of the common subtype variant Pi M2 was present with a frequency of 0.1099 in Japanese. A study of 23 twin pairs and their parents was in agreement with the hypothesis of autosomal codominant inheritance of Pi M subtypes. Other rare variant alleles, Pi MF, Pi MS, Pi MN, Pi mv, Pi MX, Pi MZ were found in very low frequencies. The total concentration of serum alpha1-antitrypsin was compared among three different phenotypic groups (M1, M1M2, M2). Statistically significant quantitative differences were found among these three groups (P less than 0.01).