Replication of simian virus 40 (SV40) DNA occurs in SV40 nonpermissive hamster cells upon infection with herpes simplex virus (HSV), leading to concatemeric replication products characteristic for HSV DNA replication. This SV40 origin (ori)-dependent process is governed by SV40 large T antigen and HSV-encoded DNA replication factors; e.g., DNA polymerase, single-strand binding protein (SSB), and helicase-primase. In this study, we show that specific interaction of SV40 T antigen with SV40 ori is crucial for HSV-directed SV40 DNA synthesis and that the property of T antigen to bind and unwind the ori is not sufficient for this process. A T antigen with the mutation T217S, affecting a hypothetical novel DNA replication subfunction, is able to support DNA synthesis in vitro but not in cultured primate cells. This subfunction is also necessary in HSV-infected hamster cells. Using temperature-sensitive mutants, we demonstrate that the T antigen acts at early stages of DNA synthesis while HSV helicase is required continuously as has been shown for HSV DNA polymerase. HSV SSB is also continuously involved in heterologous SV40 DNA synthesis. However, a HSV mutant, temperature-sensitive in SSB function, showed residual synthesis of SV40 DNA but not of HSV DNA at the nonpermissive temperature. The nature of this dichotomy between HSV SSB function on SV40 DNA and HSV DNA will be discussed.