This FRET peptide can be used to monitor selectively cathepsin K activity in physiological fluids and cell lysates. Abz-HPGGPQ-EDDnp [where Abz represents o-aminobenzoic acid and EDDnp represents N -(2,4-dinitrophenyl)-ethylenediamine], a substrate initially developed for trypanosomal enzymes, is efficiently cleaved at the Gly-Gly bond by cathepsin K. This peptide is resistant to hydrolysis by cathepsins B, F, H, L, S and V, Ex/Em=340 nm/420 nm.