A wide variety of double-stranded DNA templates are replicated extensively in an in vitro DNA replication system containing the purified proteins specified by seven T4 bacteriophage DNA replication genes (32, 41, 43, 44, 62, 45, and 61). In favorable conditions, this multiprotein system catalyzes the synthesis of several copies of the input DNA template in a 30- to 60-min incubation. The replication forks produced in vitro move in a highly processive fashion, at approximately the in vivo rate of 500 nucleotides per s. The DNA synthesized on the lagging side of the in vitro replication fork is made discontinuously, as it is in vivo, giving rise to "Okazaki pieces" averaging some 10,000 nucleotides in length; in contrast, DNA is polymerized in a continuous manner on the leading side of the in vitro fork. Although the mechanism by which the seven-protein in vitro DNA replication system propagates replication forks closely resembles the in vivo mechanism, it lacks the capacity to remove RNA primers, to reseal Okazaki pieces, and to initiate replication forks at defined DNA origins; supplementation of the system with additional T4-specific replication proteins will be required to facilitate these latter three functions.