The protein-mediated exchange of strands between a DNA double helix and a homologous DNA single strand involves both synapsis and branch migration, which are two important aspects of any general recombination reaction. Purified DNA-dependent ATPases from Escherichia coli (recA protein), Ustilago (rec 1 protein) and phage T4 (uvsX protein) have been shown to drive both synapsis and branch migration in vitro. The T4 gene 32 protein is a helix-destabilizing protein that greatly stimulates uvsX-protein-catalysed synapsis, and the E. coli SSB (single-strand binding) protein stimulates the analogous recA-protein-mediated reaction to a lesser degree. One suspects that several other proteins also play a role in the strand exchange process. For example, a DNA helicase could in principle accelerate branch migration rates by helping to melt the helix at the branch point. The T4 dda protein is a DNA helicase that is required to move the T4 replication fork past DNA template-bound proteins in vitro. Previously, we have shown that the dda protein binds to a column that contains immobilized T4 uvsX protein. We show here that this helicase specifically stimulates the branch migration reaction that the uvsX protein catalyses as a central part of the genetic recombination process in a T4 bacteriophage-infected cell.