The measurement of activities from individual proteases in biological samples is difficult because of the numerous proteases, their overlapping activities, and the lack of specific substrates. We applied selective protease inhibitors based on bicyclic peptides (>2000-fold selective over related proteases) to block individual proteases, allowing the quantification of their net activities. In protease mixtures, activity contributions of the serine proteases plasma kallikrein and urokinase-type plasminogen activator (uPA) were accurately quantified. In a tumor extract, we could quantify uPA activity. Because bicyclic peptide inhibitors toward virtually any protease can be generated by phage display, the approach should be applicable to any protease.