The presence of a leaving group at C-4 of monobactams is usually considered to be a requirement for mechanism-based inhibition of human leukocyte elastase by these acylating agents. We report that second-order rate constants for the alkaline hydrolysis and elastase inactivation by N-carbamoyl monobactams are independent of the pKa of the leaving group at C-4. Indeed, the effect exerted by these substituents is purely inductive: electron-withdrawing substituents at C-4 of N-carbamoyl-3,3-diethylmonobactams increase the rate of alkaline hydrolysis and elastase inactivation, with Hammett pI values of 3.4 and 2.5, respectively, which indicate the development of a negative charge in the transition-states. The difference in magnitude between these pI values is consistent with an earlier transition-state for the enzymatic reaction when compared with that for the chemical process. These results suggest that the rate-limiting step in elastase inactivation is the formation of the tetrahedral intermediate, and that beta-lactam ring-opening is not concerted with the departure of a leaving group from C-4. Monobactam sulfones emerged as potent elastase inhibitors even when the ethyl groups at C-3, required for interaction with the primary recognition site, are absent. For one such compound, a 1 : 1 enzyme-inhibitor complex involving porcine pancreatic elastase has been examined by X-ray crystallography and shown to result from serine acylation and sulfinate departure from the beta-lactam C-4.