Abstract: Repertoires of first specific binding pair (sbp) members wherein each first sbp member has a chemical moiety bound covalently at an amino acid residue within the binding site are made and may be displayed at the surface of an organism such as a bacteriophage. Methods of making such repertoires may involve the provision of a population of encoding nucleic acid molecules wherein a codon encoding a selectively or preferentially modifiable amino acid is introduced in the region encoding the binding site, for instance by mutation or gene construction. First sbp member (e.g. antibodies) wherein binding to second sbp member (e.g. antigen) is enhanced in or dependent on the presence of the chemical moiety in the binding site may be selected from the repertoires.