Herein, we present the synthesis and the measurement of the inhibitory activity of novel peptidyl derivatives of α-aminoalkylphosphonate diaryl esters as human neutrophil elastase inhibitors. Their selectivity against other serine proteases, including porcine pancreatic elastase, chymotrypsin, and trypsin, was also demonstrated. We also describe the preparation of single peptide diastereomers. The most active and selective compound developed possessed a k(inact)/K(I) of 2353000 M(-1) s(-1), which is the most potent irreversible peptidyl inhibitor of human neutrophil elastase reported to date. The peptidyl inhibitors were demonstrated to be stable in PBS buffer and human plasma, as were their complexes with HNE.