A series of N-acyloxymethyl- and N-aminocarbonyloxymethyl derivatives of 2-azetidinones. 3, with different substituent patterns at the beta -lactam C-3 and C-4 positions, were designed as potential mechanism-based inhibitors for human leukocyte elastase and found to exhibit inhibitory potency and selectivity for the enzyme, (C) 2001 Elsevier Science Ltd. All rights reserved.