Magnetic iron oxide (gamma‐Fe2O3) particles were injected intravenously into four male New Zealand white rabbits. Most of these particles were phagocytized by the Kupffer cells. When the animal was placed in a magnetic field, the particles in the liver became magnetized and aligned. After removal of the external magnetizing field, the particles collectively produced a remanent magnetic field which was measured at the body surface. The strength of this field was proportional to the amount of magnetic particles present in the liver; sequential measurements thus allowed us to describe their disappearance from the liver. After each magnetization, the remanent field rapidly decayed due to particle rotation (relaxation). Since the particles were confined in phagosomes or secondary lysosomes we conclude that movements of these organelles due to cytoplasmic motion caused relaxation. Magnetic particles might therefore serve as probes for cytoplasmic motility of Kupffer cells in situ.