Pulmonary toxicity by inhalation of two coal-mine dusts containing 15 and 5% quartz was compared with that of artificial coal–quartz mixtures having the same quartz content. The natural dust mixtures were slightly toxic, while the artificial ones produced considerably more lesions and fibrosis in relation to the quartz content. Intratracheal tests carried out with sands of various origins show that certain types of quartz may be only slightly toxic at first, then they progressively gain the usual toxicity of quartz in the lungs. Quartz extracted from various coal-mine dusts by chemical attack was also very toxic. It is inferred that the absence of toxicity of coal-mine dust containing quartz is due to the presence of protective impurities on the surface of the quartz particles or to the inhibitory action of certain constituents.