In recent years it has become known that proteolytic enzymes which are normally set free in pulmonary tissue can destroy the parenchyma unless they are adequately neutralized by enzyme inhibitors. Pulmonary emphysema caused by alpha 1-antitrypsin deficiency is an example of this. In the conducting airways as well, proteolytic activity occurs especially if there is infection. An enzyme inhibitor (LMI) has recently been discovered which is not present in the blood, but which is secreted by nasal and bronchial submucosal glands. The properties and the localization of LMI will be discussed, notably in the small airways. The consequences of a possible lack of this inhibitor will be discussed.