Biocompatible Au nanoparticles with surfaces modified by PEG (polyethylene glycol) were developed in view of possible applications for the enhancement of radiotherapy. Such nanoparticles exhibit preferential deposition at tumor sites due to the enhanced permeation and retention (EPR) effect. Here, we systematically studied their effects on EMT-6 and CT26 cell survival rates during irradiation for a dose up to 10 Gy with a commercial biological irradiator (E(average) = 73 keV), a Cu-Kalpha(1) x-ray source (8.048 keV), a monochromatized synchrotron source (6.5 keV), a radio-oncology linear accelerator (6 MeV) and a proton source (3 MeV). The percentage of surviving cells after irradiation was found to decrease by approximately 2-45% in the presence of PEG-Au nanoparticles ([Au] = 400, 500 or 1000 microM). The cell survival rates decreased as a function of the dose for all sources and nanoparticle concentrations. These results could open the way to more effective cancer irradiation therapies by using nanoparticles with optimized surface treatment. Difficulties in applying MTT assays were also brought to light, showing that this approach is not suitable for radiobiology.