The invention relates to a method for selecting, isolating and/or recovering a peptide or polypeptide from a library or a repertoire of peptides and polypeptides (e.g., a display system) that is resistant to degradation by a protease such as a protease found in the GI tract or pulmonary tissue of a human. Generally, the method comprises providing a library or repertoire of peptides or polypeptides, combining the library or repertoire with a protease under conditions suitable for protease activity, and selecting, isolating and/or recovering a peptide or polypeptide that is resistant to degradation by the protease and has a desired biological activity. The selected peptides and polypeptides have utility as therapeutics, eg for treating disease or conditions of GI tract or pulmonary tissue in humans.