It has been generally accepted that genetic damage is caused by the deposition of energy within the nucleus of a cell, and the extent of this damage is approximately proportional to the absorbed dose (the linear no-threshold dose-response model).  However, recent experiments have shown that genetic alterations, cell death or transformation can occur in cells that receive no direct radiation at all – the so-called bystander effect.  The goal of this project was to convert a broad-beam Hewlett-Packard (HP) 43855B Faxitron x-ray machine into a millibeam configuration that can target small sections of a cell culture dish with a precise dose of radiation.  This millibeam provides novel capabilities for radiobiological studies into the mechanisms underlying bystander effects caused by x-rays, a low linear energy transfer (LET) radiation.  Air kerma and absorbed dose calibration factors for the HP Faxitron were developed for use in Fricke dosimetry, parallel-plate ionization chambers, Lithium Fluoride thermoluminescent dosimetry (TLD), and EBT GafChromic film to characterize the spatial distribution and accuracy of the doses produced by the Faxitron.  A new film scanning and analysis technique was developed using an off-the-shelf Canon 9950F 16-bit transmission scanner and the free ImageJ software from the National Institute of Health.  A multi-layer shield composed of lead and steel was designed to convert the 26.035 cm Faxitron x-ray beam into a millibeam that targets 10 to 60% of the area on a 9-cm diameter cell culture dish.