A body cross‐section scan phantom has been designed to study problems involved in the external detection of radioactivity in human organs. It is based upon the use of serial cross‐sectional drawings of a particular body area reproduced in metallic paint. A commercial scanning mechanism moves a small electrode over one of the cross sections, and simultaneously moves a small radioactive source through a tank of water. The entire system is aligned in three dimensions relative to the position of a scintillation detector. Each organ area drawn on the cross‐section is part of an individual scaling circuit which is activated by the moving electrode. Thus, by scanning the drawing, a summation of the counts due to activity in each of these cross‐sectional areas can be automatically registered. The three dimensional response is determined by graphically integrating the data from all the cross sections.