We report a novel method for the analysis of protein ligands using a whole molecule mutagenesis/phage display system. The cDNA for the inflammatory polypeptide C5a was used as template in a PCR reaction doped with mutagenic nucleoside triphosphates (dP and 8-oxo-2'deoxyguanosine (8-oxodG)) to allow introduction of mutations in a highly controlled manner throughout the cDNA. The resultant library of mutants was displayed on the surface of phage and functional polypeptides were selected by several rounds of selection against the cells bearing the receptor for C5a. Following selection only a limited number of residues in C5a were found to be mutated, suggesting that mutations in key residues involved in the maintenance of structure and in receptor binding had been eliminated. The selected C5a sequences had a higher affinity for receptor than wild type phage-C5a conjugates. As this method for analysing the functional characteristics of proteins does not rely on knowledge a priori of structure, it may be useful for affinity maturation or analysis in a wide range of protein ligand/receptor systems.