The effects of the RM 60 series on DNA replication systems were examined by using simian virus 40 (SV40) DNA replication system in vitro. The RM 60 series inhibited the DNA replication in the initiation step rather than in the elongation step. Polymerase$alpha$-primase activity and toposiomerase I activity study were performed subsequently, the RM 60 seies increased the polymerase $alpha$-primase activity in a low dose, but decreased the activity in a higher dose. The topoisomerase I activity was inhibited by the RM 60 series. This result suggests that the RM 60 series might inhibit some molecules which are required to establish replication forks during the initiation step of the DNA replication.