Enzyme-linked immunosorbent assay (ELISA) was initially proposed as an alternative to radioimmunosorbent test to examine antigen-antibody interactions . Later, this technique was developed and applied to the study of varied molecular interactions . In general, ELISA exhibits a low limit of detection, especially in its sandwich-like variant . There are a few variants of ELISA; the simplest are direct, indirect, sandwich, and reverse types. Nevertheless, thanks to a wide range of commercially available antibodies and kits, setting up an ELISA experiment is very flexible nowadays. In a typical ELISA, the signal (target-ligand binding) is amplified only by an enzymatic reaction. However, when M13 phages are used, the signal is amplified twice. Every five pIII-displayed ligands may interact with the target, and the binding of one phage particle via single ligand-target interaction provides about 2 000 particles of pVIII protein (per virion) ready to be detected by secondary antibodies. In this way, phages provide about 10 000 fold (four orders of magnitude) amplification of the signal, enabling detection of such a low phage amount as about 10 8 plaque-forming units (pfu, corresponding molar concentration of virions is about 10 -13 M) administered into ELISA assay. ELISA may also be applied to study affinity between target and phage-displayed ligand (e.g. peptide) .