An enzyme system prepared from Saccharomyces cerevisiae carries out the replication of exogenous yeast plasmid DNA. Replication in vitro mimics that in vivo in that DNA synthesis in extracts of strain cdc8, a temperature-sensitive DNA replication mutant, is thermolabile relative to the wild-type, and in that aphidicolin inhibits replication in vitro. Furthermore, only plasmids containing a functional yeast replicator, ARS, initiate replication at a specific site in vitro. Analysis of replicative intermediates shows that plasmid YRp7, which contains the chromosomal replicator ARS1, initiates bidirectional replication in a 100 bp region within the sequence required for autonomous replication in vivo. Plasmids containing ARS2, another chromosomal replicator, and the ARS region of the endogenous yeast plasmid 2 microns circle give similar results, suggesting that ARS sequences are specific origins of chromosomal replication. Used in conjunction with deletion mapping, the in vitro system allows definition of the minimal sequences required for the initiation of replication.