An enzyme present in extracts from calf thymus degrades specifically the RNA moiety of DNA-RNA hybrids. Other nucleic acids, such as single- or double-stranded DNA and single- or double-stranded RNA, are not affected to a comparable degree. If prepared free of the hybrid-degrading enzyme, RNA polymerase from calf thymus shows a fivefold increase in activity on denatured DNA as compared to native DNA.