Magnetopneumography is the study of the remanent magnetism of foreign intrathoracic ferromagnetic particles after magnetization by an external magnetic field. Given knowledge of the magnetic characteristics of the inhaled particles, this highly sensitive and non-invasive technique allows the measurement of lung dust loads. Many groups of workers have been examined in this way, e.g. welders, coalminers, asbestos, foundry and steel workers. Magnetopneumography also allows analysis of the distribution of aerocontaminants in the different anatomical structures and, when repeated, the study of clearance speeds and migration from site to site of such particles. Emphasis has been laid on the importance of study of the fading of the remanent magnetic signal as time elapses. This short-term phenomenon, called relaxation, seems highly significant for the study of the dynamic properties of the immediate environment of extra pulmonary particles and especially for the study of macrophage activity.