Natural double-stranded DNA and the enzyme glucose oxidase (GOD) were assembled into layer-by-layer films on electrode surface. in the presence of glucose and Fe2+ in incubation solutions, GOD in the films could catalyze the reaction of glucose and O-2, and the produced H2O2 would further react with Fe2+, generating hydroxyl radical ((OH)-O-center dot) as in the Fenton reaction, which could severely damage DNA in the films. The electrocatalytic oxidation of DNA guanines by Ru(bpy)(3)(3+) (bpy = 2,2'-bipyridine) produced by electrooxidation of Ru(bpy)(3)(2+) at the electrodes was used to detect DNA damage induced by the in situ Fenton reaction. The additional catalase (Cat) layers assembled into the films could effectively protect the DNA from damage by decomposing the H2O2 produced in situ, and only when the Cat layers were located in close proximity to the GOD layers, the protection of DNA in the inner layers was most effective. The present work provides an in vitro model system to mimic the real bioprocess in DNA damage and protection through a simple electrochemical approach combined with layer-by-layer assembly. (C) 2008 Elsevier Ltd. All rights reserved.