The effects of an intratracheal inoculation of graded doses of coal fly ash were investigated in the lungs of rats. Animals exposed to increasing concentrations of the dust did not significantly increase the wet and dry weight of lungs. Collagen contents of lungs, estimated as hydroxyproline, increased significantly with lapse in time and was dependent on the dust concentration used. The dust produced a marked macrophage response in the alveoli and a thickening of the interalveolar septa, due to cellular infiltration and proliferation of argyrophilic fibres. Irrespective of the amount of dust injected, foci of dust laden macrophages were present in the mediastinal lymph nodes at all time periods. In the bronchoalveolar lavage the number of cells increased very significantly with an increase in the amount of fly ash injected. Increase in the cellular constituents of the bronchoalveolar lavage was associated with a statistically significant increase in the lactate dehydrogenase enzyme activity and protein, contents. The data obtained suggest a dose‐related toxicity of fly ash on lungs.