Escherichia coli type 1 fimbriae are composed of subunits, each of which comprises 158 amino acids. We synthesized a copy of a 13-residue peptide, located near the NH2 terminus of the fimbrial subunit, that assumed some of the properties of type 1 fimbriae. At pH 5.5 the synthetic peptide autoassembled into fibrillar structures that resembled type 1 fimbriae except that they appeared less rigid and rodlike. A quaternary structure-specific monoclonal antibody against type 1 fimbriae recognized the synthetic peptide in the assembled but not the unassembled state. Furthermore, when the synthetic peptide was injected in its fimbrial conformation into rabbits, it evoked antibodies that reacted with type 1 fimbriae isolated from E. coli.