Purified circular DNA of SV40 or polyoma virus has been injected into unfertilized eggs of Xenopus laevis. Injected DNA initiates and completes multiple rounds of semiconservative replication while observing cellular regulatory signals. Thus replication initiation of double-stranded templates is induced after the oocyte is matured in vitro by progesterone. Only one round of replication of injected DNA is observed in a single cell cycle. When protein synthesis is inhibited unreplicated molecules continue to initiate replication at an undiminished rate, but reinitiation on previously replicated molecules is completely and selectively abolished. The DNA sequence requirements for the replication of injected DNA have been investigated. A variety of procaryotic DNA molecules and circularized fragments of SV40 or polyoma DNA replicate, regardless of whether they contain the viral origin of DNA replication. These results suggest that a specialized DNA sequence is not essential for the initiation of semiconservative DNA replication in the Xenopus embryo, nor is a specialized sequence essential for the mechanism which prevents reinitiation on a molecule which has already replicated within a cell cycle. The possibility is discussed that viral origins of replication are not valid models for the eucaryotic chromosome but are adaptations for uncoupling viral replication from the mechanism which prevents reinitiation within a cell cycle.