Monoclonal antibody (mAb) 5.5 is directed against the ligand-binding site of the nicotinic acetylcholine receptor. The epitope for this antibody is conformation-dependent, and the antibody does not react with synthetic peptides derived from the receptor sequence. We have identified a ligand peptide that mimics this conformation-dependent epitope from a phage-epitope library composed of filamentous phage displaying random hexapeptides. Among 38 positive phage clones, individually selected from the library, 34 positive clones carried the sequence Asp-Leu-Val-Trp-Leu-Leu (DLVWLL), 1 positive clone had the sequence Asp-Ile-Val-Trp-Leu-Leu (DIVWLL), and 3 positive clones expressed the sequence Leu-Ile-Glu-Trp-Leu-Leu (LIEWLL), none of which are significantly homologous with the nicotinic acetylcholine receptor alpha subunit sequence. All of these phages bind specifically to mAb 5.5. The synthetic peptide DLVWLL inhibits binding of mAb 5.5 to the related peptide-presenting phage and to the nicotinic acetylcholine receptor in a concentration-dependent manner; the IC50 value is of the order of 10(-4) M. Bioactivity of the peptide "mimotope" DLVWLL was demonstrated in vivo in hatched chickens by inhibition of the mAb 5.5 effect by the peptide. The neuromuscular block and myasthenia gravis-like symptoms that are induced in chicken by passive transfer of mAb 5.5 were specifically abolished by DLVWLL. This study shows the potential of a random peptide phage-epitope library for selecting a mimotope for an antibody that recognizes a folded form of the protein, where peptides from the linear amino acid sequence of the protein are not applicable.