A technique, with the instrumentation required, is described that allows the routine mapping of magnetic lung contamination in vivo . The instrumentation consists of (1) a patient chair which moves during the mapping between two fluxgate transducers and two magnetizing coils producing a magnetic field to magnetize the lung contaminants and (2) a minicomputer for control, calculations and reporting. Measurements of lung contamination among mild steel and stainless steel welders are reported. The resolution in magnetic field measurement is 0.1 nT, corresponding to a sensitivity better than 1 mg of magnetic contaminants in the lungs in the form of magnetic iron oxides. The magnetic properties of welding fumes produced in mild steel and stainless steel welding processes are also reported.