Time course of incorporation and the effect of 5'-triphosphate of the selective antiherpetic agent (E)-5-(2-bromovinyl)-2'-deoxyuridine (bv5dUTP) on the incorporation of dTTP and dATP into template-primers of different structure were studied in E. coli DNA polymerase I Klenow fragment enzyme-catalyzed reactions. bv5dUTP could substitute for dTTP depending on the structure of template-primer. E.g. into calf thymus DNA incorporation of bv5dUMP was around 80% of that of dTMP at 30 minutes of incubation. The analog has also inhibited dTMP incorporation, net DNA synthesis, however, was hardly affected. The substrate properties of the analog were studied with [2-14C]-labelled bv5dUTP.