Here we describe a phage vector for the display of single chain antibodies and polypeptides on the surface of filamentous M13 phage which permits facile manipulation of the valency of display. The gene encoding the polypeptide is fused to a synthetic copy of the major coat protein VIII gene (gpVIII) which permits incorporation into the phage during assembly of the filament. Here we examine the growth parameters of phage propagation on the subsequent selection of an anti-progesterone antibody fragment from a mixture of display phage. Our results suggest that the density of the polypeptides displayed on phage may be modulated by altering growth conditions. This ability to influence polypeptide display density on filamentous phage may provide a versatile approach for accessing complex libraries and the capture of weaker ligand receptor interactions by avidity, whilst the potential to access and discriminate between higher affinity interactions is not negated.