Magnetic particles such as Fe3O4 and - Fe2O3 introduced into hamster pulmonary macrophages were used as a probe for intracellular motility and the rate of phagocytosis. Magnetic particles within the macrophages produce a weak magnetic field around the cells when magnetized with a strong external magnetic field. The field decays with time with a half time less than 5-6 minutes. This decay called relaxation has been recently associated with intracellular movements of microstructures of the cells. In this paper we present the results of relaxation measurements under the influence of inhibitors of ATP synthesis. We also measured the change of relaxation rate during phagocytosis (taking up of the particles by the cells.) The inhibitors notably decreased the rate of relaxation as had been expected. The decrease in the rate of relaxation corresponded well to the decrease in intracellular ATP concentration which was measured by the luciferin-luciferase method. The change of relaxation during phagocytosis of magnetic particules by pulmonary macrophages was clearly seen.