1. Human α1-antitrypsin was isolated from three Pi M and two Pi Z subjects without alteration of its microheterogeneity. The purified proteins were labelled with either 125I or 131I by a lactoperoxidase method.
2. The disappearance rate of two types of α1-antitrypsin were studied after simultaneous injection of labelled M-protein and Z-protein into Pi M subjects.
3. The ratio of extra vascular to plasma pools of α1-antitrypsin ranged between 1·2 and 1·6 with no difference between M- and Z-protein. The mean fractional catabolic rates of M-protein and Z-protein were respectively 0·26 and 0·40 per day.
4. The difference in catabolic rate of Z- and of M-protein is too small to explain why the α1-antitrypsin content of the blood in Pi ZZ subjects is only 15% of that normally found in Pi MM subjects. The low α1-antitrypsin in Pi ZZ subjects appears mainly to be due to a low rate of biosynthesis.