The SPHERO Fluorescent Magnetic Microparticles are prepared by either staining the polystyrene core of the particles with a solution of appropriate fluorophore or by polymerizing a fluorophore in styrene in the presence of polystyrene core particles. As a result, a wide variety of fluorescent magnetic particles can be prepared ranging in size, type of fluorophore, fluorescence intensity and surface functional groups. Most of the fluorophores chosen for use in the preparation of SPHERO Fluorescent Magnetic Particles are water insoluble and therefore are very stable. These fluorophores, once incorporated into the magnetic particles, do not leach and their color and fluorescence remains stable for long periods of time under proper storage conditions.