Proteolysis (or more specifically, elastolysis) of the lung may be involved in the pathogenesis of pulmonary emphysema. To investigate the human alveolar macrophage as a potential mediator of lung damage, elastase-like esterase and protease activity was determined in these cells as well as in alveolar lavage fluid and in peripheral blood leukocytes. Bronchoalveolar lavage was used to obtain alveolar cells and fluid in normal volunteers who were divided into two groups according to cigarette smoking history, nonsmokers and smokers. Results of these studies revealed that human alveolar macrophages possess a high activity of both elastase-like esterase and protease. Furthermore, the alveolar macrophages of cigarette smokers has a significantly greater elastase-like esterase and protease activity than those of nonsmokers. When the 4- to 5-fold increase in the number of macrophages found cigaretts smokers is taken into account there was approximately 10 times more elastase-like esterase activity and 18 times more protease activity within macrophages in the alveolar spaces of cigarette smokers' lungs. This makes the alveolar macrophage a poten potential source of proteolytic enzymes in man.