The development of silicotic lesions was studied in the lungs of rats pre-exposed to a pulmonary load of coal fly ash. Exposure to quartz alone increased the wet weight, dry weight, and collagen content of the lungs. These changes were associated with an increase in the activity of lactate dehydrogenase, total proteins, and the cellularity of bronchoalveolar lavage. When the lungs of rats were pre-exposed to coal fly ash for 60 days and then exposed to quartz dust for periods similar to those used for exposure to quartz alone, the development of silicotic lesions and the laying down of collagen fibres was retarded, as judged by histopathological examination and biochemical analysis of the tissues for hydroxyproline contents. These changes in the lung tissue were associated with a significant reduction in the level of lactate dehydrogenase enzyme activity, total cell counts, and protein contents of the bronchoalveolar lavage derived from rats exposed to quartz.