Effects of ribonucleotides on cis-diamminedichloroplatinum(II) (cisplatin)-mediated DNA damage were studied by incubating pUC18 DNA with cisplatin in the presence of nucleotide, and by monitoring conformational change and sensitivity change to restriction enzyme HpaII of the DNA due to the platinum-DNA adduct formation. The cisplatin-mediated DNA damage was inhibited in a dose-dependent fashion by ATP or GTP, substantially at their physiological intracellular concentrations, and almost completely by 5 mM ATP or 2 mM GTP. The inhibitory effect of nucleotide on the platination of DNA was in the order of GTP > ATP >> CTP > UTP, and of nucleoside triphosphate > nucleoside diphosphate > nucleoside monophosphate. Nucleoside did not show any significant effect on platination of DNA. To elucidate the mechanism of the nucleotide effects on platination of DNA, interaction between ATP or GTP and cisplatin was analyzed by high-performance liquid chromatography. The results suggested that ATP inhibits cisplatin-mediated DNA damage both by forming a platinum-ATP adduct and by non-covalent ionic interaction with cisplatin, while GTP acts largely by forming platinum-GTP adducts.