Antibody libraries expressed on the surface of filamentous phage are proven to be a valuable tool in isolating antibodies specific for a wide variety of antigens (for review, see ref. 1 ). As it is assumed that the probability of isolating high affinity binders is related to the initial library size (2 ), the construction of large libraries representing a high diversity of molecules is a central goal of recombinant antibody technology. In addition to generate antibody libraries from na�ve B-cell repertoires (3 –6 ), germline sequences (7 ) or immunized donors (8 ), it is also possible to make use of the available information on antibody structure to generate diversity by including short stretches of random sequences in carefully chosen parts of the antibody.