A method of screening a compound which modulates dipeptidyl peptidase I (DPPI) activities comprises the steps of adding a peptide substrate of DPPI to a reaction mixture which comprises DPPI and a compound, wherein the peptide substrate of DPPI has at least 3 amino acids and binds to a binding site of DPPI in addition to the S1-S2 site and measuring the molecular weight of the substrate, wherein a change in the molecular weight of the substrate is indicative of the presence of DPPI activity.
