The effect of poisoning the macrophages with silica on the fine structure of pulmonary contusion was studied in rats. It was found that this causes an increase in the concentration of protein in the interstitial tissue and in the air spaces. It also causes a consequent increase in the amount of interstitial oedema. While the benzo-pyrones normally cause a considerable reduction both in the protein concentrations and in the amount of oedema, they do not act in these ways when the macrophages are poisoned. These results imply that the macrophages normally lyse some of the protein and that this effect is considerably enhanced by the action of the benzo-pyrones.