In Saccharomyces cerevisiae, the RAD51 and RAD52 genes are involved in recombination and in repair of damaged DNA(1-3). The RAD51 gene is a structural and functional homologue of the recA gene(4,5) and the gene product participates in strand exchange and single-stranded-DNA-dependent ATP hydrolysis by means of nucleoprotein filament formation(6-11). The RAD52 gene(12) is important in RAD51-mediated recombination(1-3). Binding of this protein to Rad51 (refs 4, 13) suggests that they cooperate In recombination. Homologues of both Rad51 and Rad52 are conserved from yeast to humans(14-16), suggesting that the mechanisms used for pairing homologous DNA molecules during recombination may be universal in eukaryotes. Here we show that Rad52 protein stimulates Rad51 reactions and that binding to Rad51 is necessary for this stimulatory effect. We conclude that this binding is crucial in recombination and that it facilitates the formation of Rad51 nucleoprotein filaments.