[en] The X ray microprobe developed at the Gray Laboratory was originally designed to produce carbon K X rays (278 eV) by electron bombardment and focus them to a few hundred nanometers spot by using a circular diffraction grating with increasing line density (zone plate). The very fine focus achieved (<0.25 μm radius spot) and the highly localised energy deposition of C_K X rays (photoelectron range <7 nm), represent unique tools to investigate modern radiobiological phenomena. Recent improvements have been directed to increase the dose rate (up to ∼6 Gy.s^-1 entrance dose averaged over a typical V79 cell) and to evaluate the possibility of using higher energy photons (Al_K of 1.48 keV). The efficiency of the microprobe system has been tested by assessing the clonogenic potential of V79 cells irradiated with C_K X ray beams of different sizes (5 and 0.25 μm radius) and investigating the relevance of the spatial distribution of cells for the bystander effect. (author)