Bacterial infections frequently,complicate pulmonary contusion and are the leading cause of death in such patients. This study evaluated the effects of pulmonary contusion alone and contusion associated with other factors on the ability to clear aerosolized bacteria from the lung. Lung bacterial clearance of Staphylococcus aureus and Klebsiella pneumoniae was studied in animals with isolated pulmonary contusion, or contusion associated with blood loss, rapid crystalloid infusion, or steroid administration. An isolated pulmonary contusion produced no impairment of the ability of the contused lung to clear either gram-negative or gram-positive organisms. The addition of acute blood loss and crystalloid infusion resulted in decreased clearance from the contused lung; steroid administration caused a marked depression in lung bacterial clearance from the noninjured lung as well. The canine model described allowed for study of regional differences in bacterial clearance. The data presented support several conclusions; (1) the contused lung is not more susceptible to bacterial infection than the normal lung: (2) acute blood loss renders the contused lung less able to clear bacteria; (3) crystalloid infusion markedly depresses lung bacterial clearance; and (4) steroids have a deleterious antibacterial effect on both contused and noncontused lungs.