In a previous paper Cutz, Bryan et al. showed that in rabbits after repetitive lung lavage high-frequency oscillatory ventilation maintained excellent gas exchange and did not cause hyaline membrane formation (J. Appl. Physiol. 55: 131-138, 1983). In contrast, conventional mechanical ventilation had poor gas exchange and extensive hyaline membrane formation and we attributed these differences to mechanical barotrauma. However, we completely overlooked the large number of granulocytes in the damaged lung. To investigate this using the same model we have used mechanical ventilation on two groups of rabbits, one with normal granulocytes, the other depleted of granulocytes by pretreatment with nitrogen mustard. The nondepleted rabbits had poor gas exchange, a substantial protein leak into the lung and extensive hyaline membranes. The depleted animals had good gas exchange, a very small protein leak and no hyaline membranes. Repletion of granulocytes from donor rabbits lead to poor gas exchange and hyaline membrane formation. It is concluded that lung lavage causes prompt margination of granulocytes which become activated by the ongoing epithelial barotrauma of conventional ventilation.