An RNAase-sensitive DNA polymerase from rat cells transformed by avian sarcoma virus has been characterized. The enzyme requires RNA for its activity, as shown by its sensitivity to RNAase with endogenous as well as exogenous DNA templates. This sensitivity is maintained after its purification by sucrose gradients and ion exchange columns. A molecular weight of about 100 000 has been estimated. This DNA polymerase requires high salt concentration for its activity, is resistant to high concentrations of phosphonoacetic acid (400 micrograms/ml), is partially inhibited by 5 mM N-ethylmaleimide, and is completely inhibited by 0.3 mM parahydroxymercuribenzoate.