The DNA-binding mode of antitumor and antiviral agents has been evaluated by electrochemiluminescence (ECL) of tris(1,10-phenanthroline)-ruthenium complex (Ru(phen)(3)(2+)) in the presence of oxalate ion in pH 7.3 Tris buffer solution. An emission of Ru(phen)(3)(2+) was observed repeatedly with a voltage above 1000mV subjected to a potential sweep from 0 to 1250mV. The addition of lambdaDNA into the solution containing 1 micro M of Ru(phen)(3)(2+) caused the decrease in the ECL intensity, which became half at a DNA concentration of 20 micro M. This is due to the binding of Delta-type of Ru(phen)(3)(2+) with DNA in the major groove of DNA. When the various concentrations of the drug were added to the solution containing 1& micro M Ru(phen)(3)(2+), the ECL intensity was not affected by the concentration of the drug in the absence of DNA. In the presence of DNA (10 micro M), however, two ECL emission patterns were observed when the concentration of the drug was varied. The pattern that the ECL intensity increased with increasing the drug concentration was observed for cisplatin, daunomycin, and DC92-B. This may have resulted from the DNA binding of the drug with a major groove site, where Ru(phen)(3)(2+) should bind. Ru(phen)(3)(2+) nonbinding to DNA might exist in the bulk solution and exhibits ECL emission. The drug exhibiting the drug-concentration-dependent ECL is classified as a drug with a major groove binding character. The addition of drugs, such as mitomycin C and duocarmycin SA, did not cause a change in the ECL intensity even in the presence of DNA. This result indicates that these drugs bind to DNA with minor groove binding. Since similar trends were observed for actinomycin D, distamycin A, doxorubicin, and chromomycin A3; these drugs are also considered as minor groove binding agents. All these results demonstrate that the DNA-binding mode of the drug can be evaluated easily by utilizing the ECL of Ru(phen)(3)(2+), which is used as the sensing probe.