The effects of cobaltous chloride on E. coli RecA protein function and DNA strand exchange activity were examined to understand to mode of action of cobaltous chloride known as bioantimutagen. The results demonstrated two distinct effects of cobaltous chloride on the RecA protein function necessary for the strand exchange reaction. Firstly, cobaltous chloride increased the ability of RecA protein to displace SSB protein from single-stranded DNA. Secondly, cobaltous chloride enhanced the duplex DNA-dependent ATPase activity of RecA protein. We analyzed using gel electrophoresis to determine whether this enhanced activity of RecA protein represents the ability of RecA protein to catalyze strand exchange. At 10mM magnesium chloride, joint molecules containing single-stranded DNA and linear dupelx DNA were converted to product molecules with nicked circular heteroduplex DNA more rapidly in the presence of cobaltous chloride than in its absence, indicating that cobaltous chloride is able to enhance the amount of heteroduplex DNA formation in the DNA strand exchange.