The pulmonary toxicity of illite (12173603) and kaolin (1332587) dust was studied in rats. Female Wistar-rats inhaled 300mg/m3 respirable illite, kaolin, quartz (14808607), or a low rank coal dust 5 hours/day, 5 days/week for 3 months. Other rats were administered a single intratracheal injection of 12.5 milligrams (mg) quartz, 12.5mg quartz plus 37.5mg illite, or 12.5mg quartz plus 37.5mg kaolin. Selected rats were killed at 6, 12, 18, or 24 months and the lungs were removed and weighed. The left lung was taken for histopathological examination. The right lung was used for determining the collagen content and coal, quartz, kaolin, and illite burdens. In the inhalation experiment, coal dust accumulated in the lung to the greatest extent, followed by illite, kaolin, and quartz in that order. Some of the coal and quartz dusts were cleared from the lungs. None of the illite or kaolin dust was cleared. Lung weights and collagen concentrations were significantly elevated in quartz exposed rats. These parameters were not significantly affected by coal, illite, or kaolin. After intratracheal injection 48 and 37% of the dose was still present after 6 and 24 months, respectively, in the lungs of quartz exposed rats. Illite and kaolin did not significantly affect lung retention of quartz. Lung weights were significantly elevated in rats injected with quartz or quartz plus illite. Lung weights were only slightly increased in quartz plus kaolin treated rats. Lung collagen in quartz treated animals was significantly elevated only at 18 months. Kaolin plus quartz sharply increased lung collagen content at all time points. Illite tended to inhibit the increases in collagen content induced by quartz. The authors conclude that kaolin and illite behave differently in the lung. The complex nature of the mechanisms of coal dust pulmonary toxicity is probably due, at least in part, to the fact that it contains quartz, illite, and kaolin.