The functional interaction of simian virus 40 (SV40) large tumor antigen (T antigen) with DNA polymerase alpha (pol alpha)-primase complex, human single-stranded DNA binding protein (HSSB), and DNA polymerase delta (pol delta) holoenzyme, which includes pol delta, activator I (also called replication factor C), and proliferating cell nuclear antigen, at the replication fork was examined using the purified components that support SV40 DNA replication. Dilution of reaction mixtures during RNA primer synthesis revealed that T antigen remained associated continuously with the fork, while the pol alpha-primase complex dissociated from the complex during oligoribonucleotide synthesis. T antigen unwound duplex DNA from the SV40 core origin at a rate of 200 base pairs/min. Pol alpha-primase complex inhibited the rate of the unwinding reaction, and HSSB, pol alpha, and primase were all required for this effect. These requirements are the same as those essential for DNA primase-catalyzed oligoribonucleotide synthesis (Matsumoto, T., Eki, T., and Hurwitz, J. (1990) Proc. Natl. Acad. Sci. U. S. A. 87, 9712-9716). This result suggests that the pol alpha-primase complex interacts with T antigen and HSSB during the unwinding reaction to synthesize RNA primers and that the interaction decreases the rate of T antigen movement. While pol delta holoenzyme can elongate primed DNA chains at a rate of 400-600 nucleotides/min on singly primed phi X174 DNA, the rate of the leading strand synthesis catalyzed by pol delta holoenzyme in the SV40 replication system in vitro was about 200 nucleotides/min. This rate was similar to the unwinding rate catalyzed by T antigen. Thus, the rate of leading strand synthesis catalyzed by pol delta holoenzyme in vitro appears to be limited by the unwinding reaction catalyzed by T antigen.