Filamentous phage particles have been central in the construction of libraries displaying vast numbers of random peptides. These random peptides can be antigenically presented as fusions to coat proteins III and VIII of the phage. The isolation of ligate-reactive phage from an immense background of nonspecific phage is achieved by the biopanning process. Enrichment of reactive phage relative to unreactive phage occurs with alternate rounds of affinity selection to the desired molecular target and amplification of the specifically bound phage. This allows the isolation of rare binding species contained in the phage peptide libraries. Each phage particle contains the information in its genome pertaining to the type of random peptide insert displayed. Hence, the identification of binding motifs displayed on ligate-reactive phage is revealed by sequencing the relevant insert site in the phage genome. Phage peptide libraries have been used to isolate ligands to an array of protein ligates. The libraries have proved particularly effective in defining the binding sites of monoclonal antibodies and to some extent polyclonal sera. The analysis of the peptide insert sequences of a number of different clones of antibody binding phage can reveal a great deal about the nature and restriction of the amino acid residues critical for the antibody-antigen interaction.