Human leukocyte elastase (HLE) has been demonstrated on lung elastic fibers in areas of pulmonary emphysema. In vitro studies in our laboratory have shown that HLE-elastin complexes may be remarkably stable. We tested the possibility that elastin-bound HLE may retain catalytic activity in the presence of inhibitors that are effective against free HLE and found: (1) alpha-1-proteinase inhibitor (alpha 1PI), antileukoprotease (ALP), and eglin C inhibited free HLE on an approximately 1:1 molar basis, measured with either 3H-elastin or a synthetic peptide substrate; (2) the ability of each inhibitor to control catalytic activity of HLE when complexed with elastin was impaired (e.g., in a 24-h assay, a 70-fold molar excess of alpha 1PI gave only 93% inhibition of HLE); and (3) a chloromethyl ketone inhibitor of HLE gave qualitatively similar results, although at the low enzyme concentrations used it was a less effective inhibitor of free and elastin-bound enzyme than were the polypeptide inhibitors. Further, we found evidence for two distinct mechanisms of inhibition of elastin-bound HLE. alpha 1PI and eglin C prevented elastin solubilization largely by enhancing net dissociation of HLE from the complexes; enzyme remaining bound to the substrate retained essentially full activity. In contrast, ALP and the chloromethyl ketone prevented elastin solubilization by binding to the complexes and inhibiting the enzyme in situ. These results may have implications regarding progressive elastin solubilization in vivo and should stimulate further investigation of enzyme activity in heterogeneous systems in which one or more reactants are insoluble.