Purified preparations of reticuloendotheliosis virus (REV) appear to lack the endogenous RNase-sensitive DNA polymerase activity present in most, if not all, avian RNA tumor viruses. Although no endogenous RNA-directed DNA polymerase could be detected in REV, we have been able to demonstrate the presence of a virion-associated DNA polymerase by employing exogenous synthetic homopolymers as template .primer. A comparison of the REV-associated DNA polymerase activity with the RNA-directed DNA polymerase of Rous sarcoma virus (RSV) reveals similarities in the preference of the enzymes to utilize certain synthetic template .primer complexes containing ribopolymers as template. For example, both enzymes prefer poly(rA) .oligo(dT),, to poly(dA) .oligo(dT),, as template .primer. In addition, poly(rC) .oligo(dG)lo appears to be efficiently utilized by the REV DNA polymerase under conditions whereby a DNA-directed DNA polymerase does not utilize this synthetic homopolymer as template.primer. Although the REV 70 S RNA genome is not transcribed by the DNA polymerase contained within virions of REV, it is as good a template. primer for the purified avian oncornavirus RNA-directed DNA polymerase as RSV 70 S RNA. Furthermore, the virion-associated REV DNA polymerase can transcribe REV 70 S RNA when oligo(dT) ll.ll is present as a source of primer. Therefore, the inability of the REV DNA polymerase to transcribe effectively the REV genome in vitro appears to reflect either some unique property of the REV enzyme or some structural feature of REV 70 S RNA, such as the lack of certain primer molecules required by the REV DNA polymerase for the initiation of DNA synthesis.