Time-dependent inhibitors of the enzyme human leukocyte elastase have been developed based on the cephem nucleus. A series of cephalosporin tert-butyl esters has been examined, and the activity of these compounds has been found to be very sensitive to C-7 substituents, with small, alpha-oriented, electron-withdrawing groups showing greatest activity. Additionally, the oxidation state of the sulfur atom has been found to play a role in potency, with sulfones showing considerably greater activity than the corresponding sulfides or beta-sulfoxides. The alpha-sulfoxides were inactive.