The interaction of a quercetin nickel(II) complex with DNA was investigated using UV-vis spectra, fluorescence measurements, viscosity measurements, agarose gel electrophoresis and thiobarbituric acid-reactive substances assay. The results indicate that the quercetin nickel(II) complex can intercalate into the stacked base pairs of DNA, and compete with the strong intercalator ethidium bromide for the intercalative binding sites with Stern-Volmer quenching constant K(sq) = 1.0. The complex successfully promotes the cleavage of plasmid DNA, producing single and double DNA strand breaks. The amount of conversion of supercoiled form (SC) of plasmid DNA to the nicked circular form (NC) depends on the concentration of the complex, ionic strength and the duration of incubation of the complex with DNA. The maximum rate of conversion of the supercoiled form to the nicked circular form at pH 7.2 in the presence of 100 microM of the complex is found to be 0.76 x 10(-4) s(-1). The hydrolytic cleavage of DNA by the complex was supported by the evidence from free radical quenching and thiobarbituric acid-reactive substances assay.