The protective effect of ascorbic acid against double-strand breaks in DNA was evaluated by singlemolecule observation of giant DNA (T4 DNA; 166 kbp) through fluorescence microscopy. Samples were exposed to three different forms of radiation: visible light, gamma-ray and ultrasound. With regard to irradiation with visible light, 1 mM AA reduced the damage down to ca. 30%. Same concentration of AA decreased the damage done by gamma-ray to ca. 70%. However, AA had almost no protective effect against the damage caused by ultrasound. This significant difference is discussed in relation to the physico-chemical mechanism of double-strand breaks depending on the radiation source. (C) 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license.