The uvsX protein of bacteriophage T4 is a recA-type recombinase. This protein has previously been shown to help initiate DNA replication on a double-stranded DNA template by catalyzing synapsis between the template and a homologous DNA single strand that serves as primer. Here, we demonstrate that this replication-initiating activity of the uvsX protein greatly amplifies the snap-back (hairpin-primed) DNA synthesis that is catalyzed by the T4 DNA polymerase holoenzyme on linear, single-stranded DNA templates. Amplification requires the presence of uvsX protein, the DNA polymerase holoenzyme, T4 gene 32 protein, and a T4 DNA helicase, in a reaction that is modulated by the T4 uvsY protein (an accessory protein to the uvsX recombinase). The reaction products consist primarily of large networks of double-stranded and single-stranded DNA. With alkali or heat treatment, these networks resolve into dimer-length single-stranded DNA chains that renature instantaneously to reform a monomer-length double helix. A simple model can explain this uvsX protein-dependent amplification of snap-back DNA synthesis; the mechanism proposed makes several predictions that are confirmed by our experiments.