A vast library of peptides was constructed using the N-terminal hexapeptides of the adsorption protein (pIII) of fd bacteriophage. This library, containing 3×10⁸ recombinants, was used to identify ligands that bind to a monoclonal antibody (3-E7) specific for the N-terminal sequence of β-endorphin (Tyr-Gly-Gly-Phe). After three rounds of affinity purification (panning), 51 clones were sequenced and found to have tyrosine as the N-terminal residue and glycine as the second residue. These peptides showed binding affinities ranging from 0.35 μM to 8.3 μM, compared to 7.1 nM for a known high-affinity ligand. The results demonstrate that ligands can be identified without prior knowledge of antibody specificity. The library is also likely useful for identifying ligands for other receptors and discovering pharmacologic agents. The approach of using phage-displayed peptides allows for the selection of biologically active molecules from large populations of randomly generated sequences. This method has been used to study protein targeting and enzymatic catalysis. The study also shows that randomly generated peptide sequences are a rich source of ligands. The vector fAFF1 was used to construct the library, allowing for the expression of peptides in various positions in the N-terminal region of pIII. The library was screened against the monoclonal antibody, and the results indicate that the panning method is highly specific but does not discriminate between peptides with moderate and high affinities. The study highlights the potential of phage-displayed peptide libraries for identifying ligands and mapping antibody epitopes.A vast library of peptides was constructed using the N-terminal hexapeptides of the adsorption protein (pIII) of fd bacteriophage. This library, containing 3×10⁸ recombinants, was used to identify ligands that bind to a monoclonal antibody (3-E7) specific for the N-terminal sequence of β-endorphin (Tyr-Gly-Gly-Phe). After three rounds of affinity purification (panning), 51 clones were sequenced and found to have tyrosine as the N-terminal residue and glycine as the second residue. These peptides showed binding affinities ranging from 0.35 μM to 8.3 μM, compared to 7.1 nM for a known high-affinity ligand. The results demonstrate that ligands can be identified without prior knowledge of antibody specificity. The library is also likely useful for identifying ligands for other receptors and discovering pharmacologic agents. The approach of using phage-displayed peptides allows for the selection of biologically active molecules from large populations of randomly generated sequences. This method has been used to study protein targeting and enzymatic catalysis. The study also shows that randomly generated peptide sequences are a rich source of ligands. The vector fAFF1 was used to construct the library, allowing for the expression of peptides in various positions in the N-terminal region of pIII. The library was screened against the monoclonal antibody, and the results indicate that the panning method is highly specific but does not discriminate between peptides with moderate and high affinities. The study highlights the potential of phage-displayed peptide libraries for identifying ligands and mapping antibody epitopes.