A series of tripeptides which contain a, a-difluorostatone residues at Pl-Pl'and span the S3'sl'subsites have been shown to be potent inhibitors of human leukocyte elastase (HLE). The tripeptides described contain the nonproteinogenic achiral residue N-(2, 3-dihydro-lH- inden-2-yl) glycine at the P2-position. This residue has previously been shown in the case of HLE to be a good bioisosteric replacement for L-proline. Of the peptides prepared, those ...