In the reaction of the anticancer drug cis-diamminedichloroplatinum(II) (cis-DDP) with DNA, bifunctional intrastrand and interstrand cross-links are formed. In this work, we show that at 37 degrees C interstrand cross-links (ICL) are labile and rearrange into intrastrand cross-links. The ICL instability was first studied with a 10 base pairs (bp) double-stranded oligonucleotide containing a unique site-specific ICL resulting from chelation of the N7 position of two guanine residues on the opposite strands of DNA at the d(GC/GC) site by a cis-diammineplatinum(II) residue. The bonds between the platinum and the N7 of guanine residues within the interstrand adduct are cleaved. In 50 mM NaCl or NaClO4, this cleavage results in the formation of monofunctional adducts which subsequently form intrastrand cross-links. One cleavage reaction takes place per cross-linked duplex in either of both DNA strands. Whereas the starting cross-linked 10 bp duplex is hydrogen bonded, the two complementary DNA strands separate after the cleavage of the ICL. Under these conditions, the cleavage reaction is irreversible allowing its rate measurement (t1/2= 29+/-2 h) and closure of monofunctional adducts to intrastrand cross-links occurs within single-stranded DNA. Within a longer cross-linked oligonucleotide (20 bp), ICL are apparently more stable (t1/2= 120+/-12 h) as a consequense of monofunctional adducts closure back to ICL. We propose that the ICL cleavage is reversible in DNA and that these adducts rearrange finally into intrastrand cross-links. Our results could explain an 'ICL unhooking' in previously reported in vivo repair studies [Zhenet al. (1993)Carcinogenesis14, 919-924].