A library of random peptide sequences was used to select peptides that inhibit an anti-idiotypic catalytic Ig, immuno-globulin (IgG) 9G4H9, withab-lactamase-like activity. This library displays cyclic heptapeptides on the surface of bac-teriophagesand ·10 9 peptides. The first selection step aimed at enriching the lib-rary in species that bind to the whole Ig molecule. The sec-ond stepwas todiscriminate peptides that bind topart of the molecule other than the active site. Selected peptides were then screened by surface plasmon resonance analysis. Those displaying measurableKd values were assayed for their ability to inhibit the catalytic Ig
