The effects of cisplatin-DNA adducts on RecA mediated strand exchange reactions were examined in vitro. E. coli RecA protein catalizes strand exchange reaction between linear duplex and homologous single stranded DNA molecules to produce nicked circular heteroduplex DNA which can be detected by agarose gel electrophoresis. Platinum binding to both of DNA substrates inhibits significantly strand exchange reaction mediated by RecA protein. The exchanged heteroduplex DNA products were drastically decreased when the linear duplex DNA was treated with cisplatin. Those were not detected when the single stranded DNA was treated with cisplatin under the same conditions. These results suggest that single stranded DNA is more sensitive to cisplatin than duplex and/or that platinum-DNA adducts in single strands are structurally different from those of duplex.