The interaction between chlorogenic acid (CGA) and antioxidants was investigated by two different parameters: radical intensity and cytotoxicity induction. ESR spectroscopy shows that CGA produced radicals under alkaline condition. The CGA radical was scavenged by 100-300-fold lower concentrations of sodium ascorbate or N-acetyl-l-cysteine (NAC), whereas the ascorbate radical was not completely scavenged by CGA. The cytotoxic activity of CGA against human oral tumor cells (HSC-2, HSG) was completely eliminated by lower concentrations of sodium ascorbate or NAC, whereas that of sodium ascorbate or NAC was only slightly reduced by CGA. The present study demonstrated that CGA induces cytotoxicity by its radical-mediated oxidation mechanism and suggests the applicability of ESR spectroscopy for the screening of drug to drug interaction.