Seven bacteriophage T4-encoded proteins reconstitute a DNA replication apparatus that catalyzes coupled leading and lagging strand DNA synthesis at a replication fork in vitro. The proteins involved are the T4 DNA polymerase holoenzyme (the products of T4 genes 43, 44/62, and 45), a helix-destabilizing (SSB) protein (gene 32 protein), and the T4 primosome which is composed of a DNA helicase (gene 41 protein) and a primase (gene 61 protein). We show here that the presence of 41 protein on the lagging strand of the fork enables the polymerase holoenzyme to catalyze leading strand DNA synthesis at a maximum rate and with high processivity. This leading strand synthesis is unaffected by the addition of either the gene 32 or the gene 61 protein; the 41 protein cannot be replaced by the dda protein, a second T4-encoded DNA helicase. When the 61 protein is added to the 41 protein to complete the primosome, Okazaki fragment synthesis on the lagging strand accompanies leading strand DNA synthesis in this system even in the absence of the 32 protein. However, the addition of 32 protein decreases the size of the Okazaki fragments made, as expected for an increase in the lagging strand polymerization rate at a fork that has coupled leading and lagging strand DNA polymerase molecules.