BACKGROUND In recent years nano-metaloxides which easily penetrate into the cells with special interest due to their higher chemical reactivity as compared to that of similar materials in the bulk form. Of particular interest are nano-TiO(2) and ZnO, which have been widely used for their bactericidal and anticancerous properties. PURPOSE The aim of the present study was to examine the bactericidal properties of nano-TiO(2) and ZnO combined with visible light on S. aureus and S. epidermitis, known for their high prevalence in infected wounds. STUDY Using the technique of electron-spin resonance (ESR) coupled with spin trapping, we examined the ability of TiO(2) and ZnO nanoparticle suspensions in water to produce reactive oxygen species (ROS) with and without visible light irradiation. The possibility of exciting these nanoparticles with visible light in order to enhance their antimicrobial activity was also tested. RESULTS Electron-spin resonance measurements revealed that ZnO and TiO(2) nanoparticles are able to produce ROS in water suspension. A remarkable enhancement of ROS production was found following illumination with blue light. In addition, illumination significantly enhanced the antibacterial activity of the nanoparticles. CONCLUSION The results suggest that nanoparticles combined with visible light can be used for sterilization purposes and may be effective for treating infected wounds.