Analogues of araNTPs carrying an azido or aminogroup instead of the 2' hydroxyl exhibited substrate properties towards several mammalian and viral DNA polymerases. At the same time, introduction of a bulky hydrophobic DNP group into the 2' position inactivated the compounds as substrates. HSV-1 and CMV DNA polymerases were an interesting exception: they effectively incorporated the modified nucleotide residues with DNP group into the 3'-termini of the DNA chain. This is a reliable way to distinguish these enzymes from cellular DNA polymerases.