Elastase and cathepsin G are two of the major enzymes present in and secreted by human neutrophils. These proteinases can rapidly degrade connective tissue proteins. However, they also may be involved in other processes, including the activation or inactivation of protein hormones and the inactivation of plasma proteinase inhibitors. Neutrophil elastase has been implicated in the development of pulmonary emphysema, although a function for cathepsin G has not yet been elucidated. Both enzymes are normally tightly controlled by plasma proteinase inhibitors. However, this proteinase-proteinase inhibitor balance can be perturbed in favor of free enzyme by several methods, with resulting tissue damage. The use of inhibitors from several sources should be helpful in augmenting natural levels so that homeostasis can be maintained.