DNA polymerase from RNA tumor viruses ("reverse transcriptase") has been analyzed for activities which have been associated with other DNA polymerases. Homogeneous DNA polymerase from avian myeoblastosis virus catalyzes pyrophosphate exchange and pyrophosphorolysis. Pyrophosphate exchange is dependent on a template and is base-specific. With avian myeloblastosis virus DNA polymerase, ribonucleotide templates are more efficient for synthesis while deoxyribonucleotide templates are more effective for pyrophosphate exchange. Synthesis, pyrophosphate exchange, and pyrophosphorolysis were inhibited by the chelating agent 1,10-phenanthroline, suggesting that enzyme-bound zinc is required for each of these reactions. The pyrophosphate exchange reaction was also demonstrated with the DNA polymerase from a mutant of Rous sarcoma virus that possesses a temperature-sensitive DNA polymerase. The pyrophosphate exchange reaction with the mutant polymerase is temperature-sensitive which demonstrates that pyrophosphate exchange is indeed catalyzed by the viral DNA polymerase and that the same mutation effects both DNA polymerase and pyrophosphatase activity. Unlike Escherichia coli DNA polymerase I, the DNA polymerase from avian myeloblastosis virus fails to degrade polydeoxyribonucleotides or to convert deoxynucleoside triphosphates into monophosphates. This lack of hydrolytic activities in avian myeoblastosis DNA polymerase should facilitate kinetic studies on the mechanism of DNA synthesis by this enzyme.