AbstractTriggered polymorphonuclear leucocytes (PMNL) can decrease the elastase inhibitory capacity of serum by inactivating the main inhibitor of elastase alpha‐1‐proteinase inhibitor (alpha‐1‐PI). Maximal inactivation occurs with stimuli that release myeloperoxidase from PMNL along with hydrogen peroxide. Specific protection of alpha‐1‐PI function is obtained with antioxidants that interfere with this system. PMNL that are activated with phorbol myristate acetate release hydrogen peroxide but not myeloperoxidase, and only inactivate alpha‐1‐PI in the presence of exogenously‐added PMNL‐derived supernatants which contain this enzyme. Cell‐free inactivation requires both active enzyme and hydrogen peroxide, and is greatest at pH 6·2, the pH optimum for myeloperoxidase‐catalysed inactivation of alpha‐1‐PI. This data supports the notion that leucocyte myeloperoxidase may act to suppress the antiprotease screen afforded by alpha‐1‐PI by generating hypochlorous acid in the presence of chloride and respiratory burst‐derived hydrogen peroxide, and in the microenvironment of lowered pH associated with degranulation. Pulmonary emphysema seems to be associated with an imbalance between elastase and its inhibitors at the lung surface. PMNL are likely to play an important role in the pathogenesis of emphysema since they contain both elastase, which can solubilize connective tissue elastin, and the constituents of an oxidative system which can inactive the most