The results presented in this paper indicate that the phi 29 DNA polymerase is the only enzyme required for efficient synthesis of full length phi 29 DNA with the phi 29 terminal protein, the initiation primer, as the only additional protein requirement. Analysis of phi 29 DNA polymerase activity in various in vitro DNA replication systems indicates that two main reasons are responsible for the efficiency of this minimal system: 1) the phi 29 DNA polymerase is highly processive in the absence of any accessory protein; 2) the polymerase itself is able to produce strand displacement coupled to the polymerization process. Using primed M13 DNA as template, the phi 29 DNA polymerase is able to synthesize DNA chains greater than 70 kilobase pairs. Furthermore, conditions that increase the stability of secondary structure in the template do not affect the processivity and strand displacement ability of the enzyme. Thus, the catalytic properties of the phi 29 DNA polymerase are appropriate for a phi 29 DNA replication mechanism involving two replication origins, strand displacement and continuous synthesis of both strands. The enzymology of phi 29 DNA replication would support a symmetrical model of DNA replication.