Acute hemorrhagic alveolitis was elicited in the lungs of guinea pigs by aerosol challenge with sonicated P. aeruginosa whole cells. Histological findings showed the severe inflammatory changes, which were characterized by the inflammatory infiltration of alveolar macrophages (AM), polymorphonuclear leukocytes (PMN) and eosinophils in the peribronchial and the alveolar space at 8 hours after the challenge. A marked increase of PMN as well as AM was noted in the bronchial alveolar lavage fluid (BALF) at 8 hours after the challenge and the increase of the numbers of AM and PMN in the BALF remained until 24 hours after challenge. The complement titer (CH50) and C3 component in the serum decreased at the early stage, and CH50 maintained low level and C3 component increased gradually. Polyethyleneglycol precipitation-complement consumption test (PEG-CC) of the BALF showed the existence of immune complexes formed in the airway after aerosol inhalation. These data suggest that the immune complexes of P. aeruginosa activate the complement system in the lungs, which is followed by the infiltration of inflammatory cells. Guinea pigs which were pretreated with cobra venom factor significantly reduced the extent of the inflammatory changes in the lungs. The results suggests that the complement system might act as an important factor of the acute lung injury in this model.