An improved method for selecting a molecule such as an antibody, antigen, peptide, protein or fragment thereof and its encoding sequence, which molecule is expressed together with a phage coat protein on the phages surface. The improvement is achieved through a new mutant filamentous helper phage which has retained the gene III promoter, whereas the gene III encoding sequence is deleted. Amplification of phage titer of 106 times were achieved in M13-derived phages, when used for the selection of specific antibody.