Virazole (1-beta-d-ribofuranosyl-1,2,4-triazole-3-carboxamide) is a highly soluble new synthetic nucleoside having significant, reproducible activity against a broad spectrum of deoxyribonucleic acid and ribonucleic acid viruses in vitro. The drug inhibited viral cytopathogenic effects in monolayers of cells infected for 3 days with type 3 adeno, types 1 and 2 herpes, myxoma, cytomegalo, vaccinia, infectious bovine rhinotracheitis, types 1A, 2, 8, 13, and 56 rhino, types 1 and 3 parainfluenza, vesicular stomatitis, subacute sclerosing panencephalitis, Semliki Forest, Newcastle disease, and measles viruses. Hemagglutinin production by influenza A(2), influenza B, and type 1 parainfluenza viruses in chicken embryo cells was reduced by Virazole treatment. Recoverable intra- and extracellular virus titers were reduced by the drug in experiments with type 1 herpes, vaccinia, type 3 parainfluenza, and vesicular stomatitis viruses. Plaque formation by type 1 herpesvirus was also inhibited by exposure of the infected cells to Virazole. Pretreatment of cells with the compound, followed by its removal before addition of type 1 herpesvirus, severely lessened the antiviral activity; the compound was still moderately effective in reducing the viral effects on the cells when added as long as 22 hr after the virus. Parallel experiments, in which the antiviral activity of a number of known active drugs was compared, indicated Virazole to have at least a comparable degree of activity, and it was also active against a wider variety of viruses than any of these known active materials. The CCED(50) of Virazole to chicken embryo cells was approximately 1,000 mug/ml, although concentrations as low as 10 mug/ml caused slight (15%) inhibition in total cellular protein after 72 hr of incubation.