A protein from mitotic Saccharomyces cerevisiae cells that catalyzes homologous pairing and strand exchange was analyzed for the ability to catalyze other related reactions. The protein was capable of renaturing complementary single-stranded DNA as evidenced by S1 nuclease assays and analysis of the reaction products by agarose gel electrophoresis and electron microscopy. Incubation of the yeast protein with complementary single-stranded DNA resulted in the rapid formation of large aggregates which did not enter agarose gels. These aggregates contained many branched structures consisting of both single-stranded and double-stranded DNA. These reactions required stoichiometric amounts of protein but showed no ATP requirement. The protein formed stable complexes with both single-stranded and double-stranded DNA, showing a higher affinity for single-stranded DNA. The binding to single-stranded DNA resulted in the formation of large protein:DNA aggregates. These aggregates were also formed in strand-exchange reactions and contained both substrate and product DNAs. These results demonstrate that the S. cerevisiae strand-exchange protein shares additional properties with the Escherichia coli recA protein which, by analogy, gives further indication that it might be implicated in homologous recombination.