The toxicity of various minerals (kaolin, muscovite, illite) was compared by tracheal tests on rats to that of reference dusts (quartz, titanium dioxide, coal). These minerals have various degrees of toxicity. Owing to their sometimes very high concentration in the respirable dust, their toxicity should be taken into consideration. In order to define the aggravating role of quartz more accurately, dust inhalation tests were also conducted on rats with coal and a coal-quartz mixture containing about 10% quartz. The difference in the toxicity of these two types of dust which is practically not detectable up to 12 months, then increases progressively after that period. In addition, in rats having inhaled either coal or a quartz-coal mixture for 24 months, there appear tumoral lesions, principally of epidermoid type.