This paper describes a method for the preparation of Cs-137 labeled mordenite-coated spheroidal alumina particles [similar to 0.7 mm (phi)] for their utility in the computer automated radioactive particle tracking technique for the calibration of detectors. Mordenite was coated successfully, in situ, on alumina microspheres under hydrothermal conditions and characterized by X-ray diffraction, scanning electron microscopy, and energy-dispersive X-ray spectroscopy analyses. A thorough optimization of experimental parameters such as pH of the adsorbing solution, reaction time, carrier concentration, reaction volume, and temperature, etc., were carried out to arrive at the conditions resulting in optimum impregnation of Cs-137 activity into the microspheres. Under the optimized conditions, about 2.7-3.1 MBq (73-85 mu Ci) of Cs-137 could be impregnated on individual microspheres in a reproducible manner. The radioactive microspheres were subsequently covered with a thin layer of polystyrene to prevent the dispersion of Cs-137 activity during use.