1. The quantification of α1-antitrypsin (α1AT) by standard immunological techniques is altered by interaction of the protein with leucocyte elastase.

2. The results obtained for α1-antitrypsinleucocyte elastase mixtures in the presence of a functional excess of the inhibitor were relatively accurate for the first 6 h. However, continued incubation for more than 24 h led to a major over-estimation of the α1AT as the result of breakdown of the enzyme-inhibitor complex releasing a partially proteolysed form of the inhibitor.

3. In the presence of excess enzyme up to a twofold overestimation of α1AT occurred within 1 h and the degree of overestimation increased with time (up to threefold at 24 h). This was eventually associated with the presence of only a partially proteolysed form of α1AT (mol wt. ⋍ 50000).

4. Different results for each sample were obtained when different polyclonal antisera were used to quantify the α1AT.

5. Complete inactivation of α1AT by oxidation resulted in little change in the immunological quantification of the protein. However, further addition of H2O2 led to a progressive underestimation of the α1AT.

6. The effect of physicochemical alteration on the immunological quantification of α1AT by different antisera should be borne in mind for all studies assessing this protein in lung secretions.

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