The display of antibody fragments on the surface of filamentous bacteriophages (1 –7 ) constitutes a powerful system for the selection of molecules with desired specificities. In phage display, the antibody fragment is coupled to the minor coat protein (protein3) of bacteriophage M13 phage and is, in this way, both anchored in the phage capsid and exposed on the phage surface, linking specificity and genetic information. This permits direct isolation and sequence determination of the gene encoding the antibody fragment. The gene can subsequently be exposed to further engineering and selection in order to improve affinity and specificity.