AbstractThe activity of porphobilinogen deaminase (PBGD), an enzyme whose partial deficiency is associated with acute intermittent porphyria (AIP), changes during development. Little is known about the postnatal change of PBGD activity and the prevalence of its electrophoretic variant in the Japanese population.The activity of PBGD was measured fluorometrically in 194 infants aged 0–12 months, while isoelectric focusing of PBGD was performed in 400 healthy Japanese adults aged 20–45 years and 30 children with various hematological disorders aged 1–15 years.The PBGD level was 1.9 times higher in the neonates than in the adults, decreased abruptly during the first month of life, and reached the adult level at the age of 9 months. None of the 400 healthy Japanese adults and the 30 children with hematological disorders showed any electrophoretic variant.These results suggest that there is no need to consider any polymorphism in the gene dose study of PBGD and that the biochemical screening of AIP is applicable to since the late in