Single-stranded DNA binding protein (SSB) is well-characterized as having a helix-destabilizing activity. The helix-destabilizing capability of SSB has been re-examined in this study. The results of restriction endonuclease protection assays and titration experiments suggest that the stimulatory effect of SSB on strand exchange acts by melting out the secondary structure which is inaccessible to RecA protein binding; however, SSB is excluded from regions of secondary structure present in native single-stranded DNA. Complexes of SSB and RecA protein are required for eliminating the secondary structure barriers under optimal conditions for strand exchange.