Summary5-Iodo- and 5-bromo-2′-deoxy-uridine, when tested in the agar-diffusion plaque-inhibition test, were found to inhibit plaque formation of DNA containing vaccinia and herpes simplex viruses but not RNA containing West Nile and Newcastle disease viruses. A number of other compounds that might be candidates for the disruption of normal nucleic acid biosynthesis, including aminopterin and 5-fluoro-2′-deoxyuridine, also were tested and found inactive in this test system. Thymidine readily reversed plaque inhibition by both 5-iodo- and 5-bromo-2′-deoxyuridine.