Amino acid-derived phthalimide and saccharin derivatives have been investigated for their inhibitory activity toward the serine proteinases human leukocyte elastase, cathepsin G, and proteinase 3. The saccharin derivatives were found to be effective time-dependent inhibitors of elastase and proteinase 3 (kobs/[I] values ranged between 180 and 3620 M-1 S-1) and showed weak or no inhibition toward cathepsin G. The corresponding phthalimide derivatives were found to be inactive.