The specific function of poxviruses DNases is not completely understood. Two roles have been postulated for these activities, one in replication and processing of the viral genome, the other in the inhibition of host DNA synthesis. As a direct extension of previous work and current studies, it is planned (1) to reexamine the process of poxvirus DNA replication with emphasis on the role of the virion-associated DNases and to establish the sites of initiation and termination of replication in the viral chromosome (for this purpose, recently isolated temperature sensitive mutants of vaccinia, defective in DNA synthesis, will be employed along with wild-type virus), (2) to investigate the transitory stimulation of host DNA synthesis with Shope fibroma virus, an agent involved in the etiology of benign tumors, and (3) to develop a cell-free system to study the synthesis of virus codified proteins, including the virus-associated nucleases. It now seems obvious that in-depth understanding of DNA replication must provide significant clues to the process of malignancy. Information sought in this project could explain the role of DNases in DNA replication and virus cytopathology, and mechanisms that control the expression of the viral genome.