Abstract: A method of identifying a peptide which permits or facilitates the transport of an active agent through a human or animal tissue. A predetermined amount of phage from a random phage library or a preselected phage library is administered in vivo or in situ to a site in an animal, such as into the gastro-intestinal tract. At a predetermined time, the phage which is transported across a tissue barrier is harvested at a harvesting site, such as in portal or systemic blood or brain tissue, which is separated from the site of administration by the tissue barrier to select transported phage. This transported phage is amplified in a host. This cycle of events is repeated (using the transported phage produced in the most recent cycle) a predetermined number of times to obtain a selected phage library containing phage which can be transported from the site of administration to the harvesting site.