We have produced monoclonal antibodies (MAbs) which react with a minor and the major coat proteins of filamentous phage M13 and have characterised them by combining the techniques of enzyme-linked immunosorbent assay (ELISA) and Western blotting. These coat proteins are the minor coat protein, gIIIp, the product of gene III and the major coat protein, gVIIIp, the product of gene VIII. Both gIIIp and gVIIIp are important in the context of 'phage display' of foreign peptides/proteins as fusions to these proteins. The anti-gIIIp MAbs were able to detect native gIIIp as well as the fusion proteins comprising foreign proteins and gIIIp in ELISA and on Western blot, indicating their utility for studying the expression of foreign proteins in phage display. Similarly anti-gVIIIp MAbs detected gVIIIp both in ELISA and on Western blot. In an 'affinity capture phage ELISA', phages that were captured by virtue of the interaction between the foreign protein (ligand fused to the gIIIp and displayed on the phage surface) and the immobilised counterpart (receptor), could be easily detected using anti-gVIIIp MAbs. Considering the potential of 'phage display' technology in protein engineering, these antibodies should find wide applications.