Enzymatic activities capable of degrading double-stranded RNA have been solubilized from whole 9-day-old chick embryos and separated by ion exchange chromatography on DEAE-cellulose into two classes, designated nucleases DI and DII. Nuclease DI exhibits an absolute requirement for Mn2+ in the range of 5 to 10 mM. Monovalent cations, including K+, Na+, and NH4+, are inhibitory. The molecular weight of DI is 60,000 to 62,500 as estimated from sedimentation in sucrose density gradients. Following gradient fractionation, nuclease DI possesses the ability to degrade several substrates exhibiting a 250-fold preference for poly(rC) as compared to poly(rC)-poly(rG). The activity responsible for degrading double-stranded RNA functions as an endonuclease generating oligonucleotides with 5'-phosphate termini. Nuclease DII requires both monovalent and divalent cations. Optimal degradation of poly[r(A-U)] is seen at 75 to 100 mM salt and 0.5 to 1.0 mM MgCl2 or MnCl2. The molecular weight estimated from sucrose gradient sedimentation is in the range of 38,000 to 40,000. Nuclease DII acts endonucleolytically producing oligonucleotides terminating in 5'-phosphates. During the isolation and characterization of nucleases DI and DII, a third activity was detected which degrades single-stranded RNA substrates but which, in the presence of either DII or RNase H, significantly enhances the degradation of poly[r(A-U)] or poly(rA)-poly(dT) substrates.