The protein kinase pUL97 of human cytomegalovirus plays important but incompletely defined roles in viral replication. Concerning the early phase of infection, it is postulated that pUL97 possesses regulatory functions in gene expression and/or DNA synthesis. Here we report that pUL97 interacts with an essential component of the replication complex, the DNA polymerase processivity factor pUL44. Interaction was determined by yeast two-hybrid and coimmunoprecipitation analyses and was mapped to the pUL97 region 366-459. In vitro kinase assays demonstrated that pUL44, coimmunoprecipitated either from transfected or from infected cells, is phosphorylated by pUL97 (but not by a catalytically inactive pUL97-mutant). In infected fibroblasts, immunofluorescence analysis revealed that pUL97 and pUL44 accumulate in the nucleus and are both incorporated into viral replication centers. The treatment with inhibitors of DNA synthesis or pUL97 kinase activity largely prevented colocalization. Thus, pUL97 may be indirectly involved in viral genome replication by modifying the replication cofactor pUL44.