Pyrazofurin (PF) (3,β-D-ribofuranosyl, 4-hydroxyprazole-5-carboxamide) is a C-nucleoside antibiotic, one in which the ribose joins the base-ring carbon instead of a base-ring nitrogen. Pyrazofurin potently inhibits orotidine 5'-monophosphate (OMP) decarboxylase, thereby interfering with de novo synthesis of uridine nucleotides and resulting in cytotoxicity. This agent also causes a rapid depletion of the pyrimidine deoxynucleotide pool, thereby inhibiting DNA synthesis and cell replication. PF was isolated from fermentation filtrate of Strepomyces candidus. This compound was initially found to have inhibitory activity against the vaccinia, herpes simplex, rhino and measles viruses in vitro and the vaccinia virus in vivo. More recently the antiviral spectrum has been extended to include the polio, Coxsackie, Sindbis and vesicular stomatitis viruses.