Two DNA polymerases purified from normal human lymphocytes are distinguishable from the viral reverse transcriptases of avian myeloblastosis virus and Mason-Pfizer monkey virus by their relative affinity for select templates. In this respect, the activity of the two normal human lymphocyte polymerases closely resembles the activity of Escherichia coli DNA polymerase 1. The viral and cellular DNA polymerases are equally active with the nonspecific template, poly(rA) . poly(dT). Criteria for distinguishing the activity of viral reverse transcriptase are discussed.

Visna virus 70S RNA contains long stretches of polyadenylic acid [poly(A)]. The homogeneity in length of poly(4) regions is observed in 70S RNA from visna virus and all RNA tumor viruses tested, and not with other types of RNA. By this criterion visna virus resembles RNA tumor viruses.

Polyuridylic acid inhibited DNA polymerases purified from three species of oncornaviruses as well as three out of seven DNA polymerases purified from cells. Viral and cellular DNA polymerases could not be distinguished by polyuridylic acid inhibition, but were easily distinguished by their template preferences in the presence of magnesium.

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