Two peptides (pM and pR) from the reactive-site region of guamerin (a leech elastase inhibitor) were synthesized. Peptide pM is identical to the sequence Thr(31)-Gly(49) with methionine at the P1 site, peptide pR has the same sequence, but with arginine at P1. The inhibitory activities against elastase and subtilisin expressed only after oxidation in the presence of glutathiones. These results suggest that disulfide bridges between peptides is necessary for the activities of these peptides.