The display of repertoires of antibody fragments on the surface of filamentous phage offers a new way to produce immunoreagents with defined specificities. Phage derived antibody fragments offer a number of advantages over mouse monoclonal antibodies, such as better clearance from the blood, the possibility to select from human combinatorial libraries and the relative ease by which such fragments can be manipulated. The phage display technique thus facilitates the selection of antibody fragments of therapeutic value or research interest. Antibodies to M13 filamentous phage coat proteins are instrumental in the selection and detection of phages expressing specific antibody fragments or peptide sequences at their surface.