A 46-year-old woman suffering from rheumatoid arthritis developed numerous round opacities at the apex of the right lung 11 years after an exposure to dolomite . Resected lung showed discrete nodules, 0.8-2 cm in diameter, with central necrosis surrounded by palisading fibroblasts and a prominent inflammatory zone. A large number of birefringent dust particles were seen in the necrotic centres and around the nodules. By electron microscopy the particles were dense, mostly elongated and lamellar, varying from 0.005 to 3 microns in width, and from 0.1 to 6.5 microns in length. Energy dispersive x-ray microanalysis of the dust particles gave elemental spectra with high spikes of silicon, aluminium and potassium, and minimal magnesium, calcium, iron and titanium. According to chemical analysis, the original dolomite consisted almost entirely of magnesium and calcium carbonates and only of traces of silicon, aluminium and potassium. Apparently the human organism can better eliminate calcium and magnesium carbonates than silicon, aluminium and potassium.