A pharmacokinetic study of&agr;1-antitrypsin (ATT) was performed in 2 groups of homozygous PiZ-deficient patients (treated and untreated) and 1 group of healthy volunteers. The distribution of the131I-labelled protein corresponds to a 3-compartment model. The intravenously administered protein diffused quickly to the extravascular compartment where some retention occurred. No significant difference in AAT metabolism was observed between the 3 groups.The half-life of the injected protein is slightly longer than 2.5 days. The AAT protein was not stored. These results confirm the observations collected during the clinical trials. That is, a weekly infusion is necessary to obtain stable serum AAT concentrations. Monthly infusions are unable to maintain a ‘plateau’ phase. The periodicity may be limited to every 2 weeks.