Km and Vmax values for d(pT8) and its derivatives containing various 5′-end groups were estimated in the reaction of DNA polymerization α catalyzed by DNA polymerase α and HIV-RT. The effect of 5′-end modification of primer is more pronounced in the case of HIV-RT. Strong influence is observed for an intercalating (ethidium) group. The affinity of EtpT8 is 200-fold higher than that of d(pT8). Attachment of Phn-, Dnm- and Hem-groups results in the increase of affinity of modified primer from 10 up to 20 times. For DNA polymerase α the influence of modifiers on primer affinity is much weaker. The effect of 5′-end residues on the Vmax values is also more pronounced for HIV RT. The way to improve selective interaction of oligonucleotide derivatives with the primer site of HIV RT is suggested. © 1993 Kluwer Academic Publishers.