Neutrophil recruitment to the lung and the release of their oxygen radicals and proteolytic enzymes have been implicated in the pathogenesis of many acute and chronic lung diseases. However, the processes involved in neutrophil migration are far from clear. Directed movement of circulating neutrophils can be stimulated by a variety ofagents including the peptide FMLP. Binding of FMLP to cell membrane receptors is believed to instigate a" chemotactic" response that is dose dependent. The maximal chemotactic response occurs between 10-8 and 10-7 M FMLP (1), whereas higher concentrations of FMLP (10-6 M) result in degranulation of the neutrophil (2) and superoxide production (3). The sequence of events after FMLP receptor binding is poorly understood, although a cell surface chymotrypsinlike proteinase has been implicated. Internalization of cell surface receptors may depend on such an enzyme (4) and cell migration (5) and superoxide production (6) can be inhibited by chloromethyl ketones or monoclonal antibodies, which inactivate chymotrypsin-like enzymes. Further support for the role of chymotrypsin-like enzymes has been provided by studies with naturally occurring inhibitors. For instance,