Purified preparations of frog virus 3 possess ribonuclease activities directed against single-and double-stranded RNA. Double-stranded RNAs isolated from purified reovirus type 3 and from HeLa cells infected with poliovirus and single-stranded poliovirus RNA from purified virus are readily degraded by incubation with frog virus 3. The mode of action of the nucleases is endonucleolytic. Under the assay conditions used for the viral enzyme, crude extracts of uninfected HeLa, L, and baby hamster kidney cells did not show enzyme activity against double-stranded RNA but exhibited activity against single-stranded RNA. The dependence of the viral nucleases on divalent cations for optimal activity and the inhibition of the cleavage of single-stranded RNA by 0.2 M NaCl suggests that the enzymes are either virus-coded or virus-induced.