Phosphonoacetic acid (PAA) effectively inhibited herpesvirus saimiri (HVS) replication and the onset of virus DNA synthesis. A PAA-resistant (Pr) mutant of HVS was isolated which plaqued efficiently in the presence of concentrations of PAA sufficient to reduce the growth of wild-type virus to less than 0.02% of control values. In contrast, virus growth and DNA synthesis in cells infected with unselected strains of herpesvirus saimiri was highly resistant to concentrations of aphidicolin, an inhibitor of alpha-type polymerases, which completely inhibited the growth and DNA replication of uninfected cells. An increased level of DNA polymerase activity was induced in cells infected with herpesvirus saimiri. This HVS-induced DNA polymerase was more sensitive to PAA but more resistant to aphidicolin in vitro than the uninfected cell activity and could also be distinguished on the basis of its response to ionic strength (40 to 50 mM-ammonium sulphate for optimal activity versus 20 mM for the uninfected cell activity). Under suitable in vitro assay conditions, an increase in the PAA-resistance of the DNA polymerase induced by the HVS(Pr) mutant was demonstrated. Comparison of the effects of aphidicolin and PAA demonstrated that the former was a much more effective and rapid inhibitor of susceptible cell DNA synthesis in vivo. Taken together, these results demonstrate novel properties of a DNA polymerase activity in cells infected with herpesvirus saimiri and suggest that aphidicolin should provide a useful reagent to analyse the functions of this enzyme in productive and non-productive infections with the virus.