Productive infection of WI-38 cells with human cytomegalovirus (HCMV) induced the increase in the activity of DNA polymerases as well as the synthesis of viral and cellular DNA. Sedimentation analyses in sucrose gradients of high ionic strength showed that the HCMV infection caused marked increase in the activity of α-type polymerases (resolved into α 1 , 8 S, and α 2 , 6 S, in the present experiments), while the infection little affected the level of β-type polymerase (about 3.5 S) activity in both the nuclei and cytoplasm. Such increase in α-type polymerases was also observed when DNA synthesis in WI-38 cells was enhanced by SV40 infection or by an increased concentration of serum in medium. Phosphonoacetate, which selectively blocked the synthesis of HCMV DNA, did not significantly affect the HCMV-mediated induction of DNA polymerases. However, phosphonoacetate added in the reaction mixture for DNA polymerase assay inhibited the activity of the HCMV-induced polyperase α 1 but not of the polymerases α 2 and β. These results support the idea that α-type polymerases are involved in the replicative synthesis of cellular and viral DNA.