Rad52 protein plays a central role in double strand break repair and homologous recombination in Saccharomyces cerevisiae. We have identified a new mechanism by which Rad52 protein stimulates Rad51 protein-promoted DNA strand exchange. This function of Rad52 protein is revealed when subsaturating amounts (relative to the single-stranded DNA concentration) of replication protein-A (RPA) are used. Under these conditions, Rad52 protein is needed for extensive DNA strand exchange. Interestingly, in this new role, Rad52 protein neither acts simply as a single strand DNA-binding protein per se nor, in contrast to its previously identified stimulatory roles, does it require physical interaction with RPA because it can be substituted by the Escherichia coli single strand DNA-binding protein. We propose that Rad52 protein acts by stabilizing the Rad51 presynaptic filament.