The invention relates to the use of an iodine-2'-deoxyuridine triphosphate or substrate analogue thereof as a substrate in quantitative determination of nucleic acid polymerase activity. The invention also relates to a method of quantitatively determining nucleic acid polymerase activity, comprising measurement of the incorporation of a <125>I-labeled iodine-2'-deoxyuridine triphosphate or substrate analogue thereof into a template. The invention further relates to a kit for determining polymerase activity or its blocking antibody comprising a <125>I-labeled iodine-2'-deoxyuridine triphosphate or substrate analogue thereof, a free or immobilized template, and optionally a primer.
