A crucial event in DNA replication is the polymerase switch from the synthesis of a short RNA/DNA primer by DNA polymerase alpha/primase to the pro?cessive elongation by DNA polymerase delta. In order to shed light on the role of replication factor C (RF-C) in this process, the effects of RF-C on DNA polymerase alpha were investigated. We show that RF-C stalls DNA polymerase alpha after synthesis of approximately 30 nucleotides, while not inhibiting the polymerase activity per se. This suggested that RF-C and the length of the primer may be two important factors contributing to the polymerase switch. Furthermore the DNA binding properties of RF-C were tested. Band shift experiments indicated that RF-C has a preference for 5' recessed ends and double-stranded DNA over 3' ends. Finally PCNA can be loaded onto a DNA template carrying a RNA primer, suggesting that a DNA moiety is not necessarily required for the loading of the clamp. Thus we propose a model where RF-C, upon binding to the RNA/DNA primer, influences primer synthesis and sets the conditions for a polymerase switch after recruiting PCNA to DNA.