We have utilized liquid chromatography electrospray ionization mass spectrometry (ESI-MS) to probe the nature of the covalent E-I complex of human leucocyte elastase (HLE) and a beta-lactam. The mass spectrum of HLE isozyme 4 displayed one major and two minor components with masses of 25,202, 25,043, and 24,522 Da, respectively. Isozyme 3 displayed three components, with masses of 25,180, 24,030, and 24,523 Da. These data suggest that the isozymes differ in the type and not the content of carbohydrate. The minor components represent decreases in carbohydrate content. Inactivation of isozyme 4 with trans-4-(ethoxycarbonyl)-3-ethyl-1-[(4-nitrophenyl)sulfonyl]-azetidin -3-one increased the mass of the three components by that of the parent compound. Similar results were obtained with the mixture of HLE isozymes. These observations demonstrate that HLE does not catalyze the beta-elimination of p-nitrophenylsulfinate as Firestone et al. [(1990) Tetrahedron 46, 2255) suggested. In addition, it suggests that a "double hit" of both the active-site serine and histidine is not required to form a stable acyl-enzyme. Noncovalent complexes between HLE and either the tight-binding secretory leucoprotease inhibitor (SLPI) or a slow tight-binding peptide difluoroketone inhibitor were not observed by ESI-MS. SLPI displayed a mass of 11,710 Da in the absence and presence of HLE. These data demonstrate the utility of ESI-MS to probe the mechanism of inhibition of enzymes by mechanism-based inhibitors.