Protein A has long been the ligand of choice in the affinity purification of immunoglobulin G₁ (IgG₁) monoclonal antibodies (see Notes 1 and 2). However, current research efforts (1–9) have been focused on the discovery of small molecules (peptides or peptidomimetics) that share similar binding characteristics with protein A but are more cost effective owing to small size (for ease of synthesis) and stability (for ease of regeneration). The following methods were developed as part of a proof of principle study (1) to determine whether phage display technology could be used to identify peptides as leads in the customization of ligands for affinity chromatography (see Note 3) and (2) to identify a peptide or peptidomimetic for use as a protein A alternative in the affinity purification of monoclonal antibodies. In this study, the constant region (pFc’ fragments; see Note 4) of an IgG₁ monoclonal antibody, denoted humanized anti-Tac (HAT), was used as the target for phage display in this study. HAT is a humanized monoclonal antibody against the low-affinity p55 subunit of the interleukin-2 (IL-2) receptor.