A new method of estimation of dissociation constants for ligands and free energies of its binding based on the affinity modification of active centers in the presence of competitive ligands was developed. This method is designed for the analysis of protein-nucleic acid interactions in template systems. Deoxyoligoribonucleotides containing the reactive residue of cis-aquadihydroxydiaminoplatinum (II) and oligonucleotides ethylated at phosphate groups were used for the study of interactions of human placental DNA-polymerase alpha and the Klenow fragment of DNA-polymerase I from E. coli with templates and primers. A model was constructed which postulates the formation of a single Me2+-dependent electrostatic bond and of a hydrogen bond by one of template phosphates with the enzyme active center. Similar bonds form the basis for the enzyme interaction with the 3'-terminal phosphate group of the primer. Other monomeric units of the template are likely to interact with the enzyme by forming hydrophobic bonds. Other mononucleotide units of the primer are involved in complementary interactions with the template. The primer activity of dNMP and NMP in these systems has been demonstrated for the first time. The efficiency of dNMP, dNDP and dNTP interaction with DNA-polymerase was estimated from the affinity modification of the enzymes by dNTP and dNMP imidazolides. The key role of the template-primer interaction in the formation of the dNTP-binding site of DNA-polymerases was demonstrated. A significant contribution of dNTP gamma-phosphate to the template--dependent specific tuning of substrate dNTP was revealed.