Elastase activity directed against lung extracellular matrix is currently believed to be important in the pathogenesis of pulmonary emphysema. Although human alveolar macrophages degrade elastin when in direct contact with this substrate in vitro, studies of free elastase activity in medium conditioned by human alveolar macrophages have yielded variable results. As human alveolar macrophages secrete the tissue inhibitor of metalloproteinases (TIMP), an inhibitor of collagenase and of other connective-tissue-derived mammalian metalloproteinases, we speculated that this inhibitor's effects might extend to macrophage elastase. Using metalloproteinase elastase from the murine macrophagelike cell line P388D1, we observed that human alveolar macrophage conditioned medium inhibits metalloproteinase elastase and that this inhibitory activity could be blocked by specific antibody to TIMP. Alpha 2-macroglobulin, another proteinase inhibitor secreted by alveolar macrophages, also inhibited metalloproteinase elastase, but its inhibitory capacity was not blocked by antibody to TIMP. Because detergents are often included in elastase assays, we examined the effects of sodium dodecyl sulfate (SDS). Buffers containing SDS and SDS-treated elastin were found to exert diverse effects on metalloproteinase elastase, TIMP, and alpha 2-macroglobulin activities, including a marked inhibition of metalloproteinase elastase activity by SDS-containing buffers. These findings suggest that detection of secreted metalloproteinase elastase activity by human alveolar macrophages is complicated by the concomitant release by these cells of inhibitors of metalloproteinases, and that assay conditions can markedly influence the results.