For all phages, calculated Kd vary for different assumed complex stoichiometries. For the 1:1 stoichiometry Kd values are the lowest (highest affinity). Then, they rise for bivalent receptors and slightly decrease for pentavalent. The M13 phages usually have three to five copies of the pIII protein . Therefore, the expected valency is between 2 to 5. For every variant, the difference between Kd calculated under the receptor's mono-and bi-valency assumption is higher than the difference between 1:2 and 1:5 stoichiometry. It suggests that the accurate stoichiometry is closer to 1:2 or 1:5 than to 1:1. The determined values are very low compared to typical Kd for immunological complexes. However, the presented method requires working under similar concentrations of M13 and TnT. The linear range of concentrations obtained for phages using k-ELISA was in the picomolar range; therefore, TnT concentration also had to be picomolar. A significant disadvantage of this method is that phage titre biologically measured may differ from the real physical concentration of phage particles in solution measured during ELISA . In the biological method, phages are only detected if they can infect bacteria and induce dye (x-gal) metabolism to a blue product. In solution, phages may form aggregates that produce false low phage titres from biological assays. For A3, B7 and D11 clones, values of Kd are in a similar range which does not correlate with their distribution in the recovered pool (Table ). This fact may be explained by the varied rate of phages' amplification in bacteria culture caused by the different proportions of amino acids in peptides and the possible interaction of peptides with bacterial proteins. The same explanation is suggested by the manufacturer of the Ph.D.-12 TM kit as justification for advising against the amplification of phage-displayed peptide libraries . For the wild-type M13, it was impossible to calculate a Kd that may have a physical sense (calculated Kd values were negative), which shows that the model may be applied only for specific interactions.