Human neutrophil granulocytes contain proteolytic enzymes. In purulent bronchial and paranasal secretions these enzymes have been found extra cellulary in complex with enzyme inhibitors as well as in free form indicating saturation of the inhibiting capacity. Isolated human leukocyte enzymes, elastase and neutral protease, were found to arrest the mucociliary activity and subsequently cause superficial tissue destruction. Elastase was found to be the most potent of the enzymes. Experimental studies with elastase together with specific inhibitor indicated the importance of the enzyme inhibitors for the integrity of the mucous membrane.