Selective DNA extraction and hybridization procedures were used to estimate the relative number of covalently closed circular viral genomes in cultures of Epstein-Barr virus (EBV)-transformed cells. In virus-producing P3HR-1 cultures that were exposed for 11 days to phosphonoacetic acid or to acyclovir, the content of covalently closed circular EBV DNA was reduced ca. 70% relative to a control culture without drug. The EBV plasmid content of Raji, a virus nonproducer cell line, was not reduced by exposure to these compounds. When P3HR-1 cultures were exposed to 12-O-tetradecanoylphorbol-13-acetate, the number of circular genomes per cell increased. These findings indicate that two enzyme activities synthesize circular EBV DNA and that the virus-associated DNA polymerase synthesizes most of the circular EBV DNA in a virus producer culture. It is suggested that the circular genomes synthesized by the viral enzyme are intermediates in the syntheses of linear virus DNA.