The mechanism of binding and elongation of the oligothymidylate primers in the systems of the DNA polymerase alpha from human placenta and DNA polymerase I from E. coli with the poly(dA) as a template was investigated. Both dTMP and dTTP were shown to be the minimal primers of DNA polymerase alpha, the affinity and V increasing 1.8- and 1.4-fold respectively upon lengthening the primer by each unit from dTMP to d(Tp)9T. Further elongation is accompanied by 1.3-fold affinity enhancement and a decrease in V. For the E. coli enzyme, a similar dependence of affinity of primer d(Tp)4T-d(Tp)14T was observed with the inflexion point corresponding to d(Tp)8T. The individual diastereomers of oligothymidylate ethyl esters (with p' and p'' corresponding to enantiomeric configuration) such as d[Tp'(Et)Tp]3Tp'(Et)T, d[Tp''(Et)Tp]3Tp''(Et)T, d(Tp)8Tp'(Et)T, d(Tp)8Tp''(Et)T, d(Tp)8Tp'(Et)TpT, d(Tp)8 X X Tp''(Et)TpT and completely esterified analogues d[Tp(Et)]7T, d[Tp(Et)]14T were shown to initiate the poly (dA)-dependent polymerization catalyzed by both enzymes. A sum of the obtained results provided the basis for a number of conjectures on the mode of primer and template binding to the enzyme, possible role of their preformed complex, as well as electrostatic interactions and hydrogen bonding.