Herpes simplex virus can be eliminated from rabbit kidney tissue culture and human amnion tissue culture and vaccinia virus can be eliminated from rabbit kidney tissue culture by treatment with 5-iodo-2′-deoxyuridine in concentrations from 10 μg/ml to 1 mg/ml if the drug containing media was changed daily. This “cure” of virus infection is possible even when cytopathogenic changes are easily recognizable before treatment is begun, and such treatment results in prevention of progression of such virus induced changes. This property appears to be unique for therapeutically active anti-viral compounds and may facilitate the screening of agents active against other viruses and the more detailed study of the therapeutically active anti-viral agents now known.