Mononuclear phagocytes have the capacity to produce an array of MMPs. Several of these proteinases are capable of degrading insoluble elastin, an important component for the structural stability of the lung. Matrilysin is a low molecular weight proteinase with a broad substrate specificity produced at highest levels in in vitro differentiated monocytes. The 92-kD gelatinase is a major product of human alveolar macrophages that is also an elastase. The most newly described member of the MMP family is human macrophage metalloelastase. This enzyme is also expressed in alveolar macrophages derived from cigarette smokers. Determining the contribution of these and other elastolytic proteinases to the pathogenesis of emphysema is a focus of ongoing research.