We evaluated the usefulness of a protected specimen brush (PSB) in obtaining uncontaminated lower respiratory tract material for bacteriologic examination in a primate model of oleic acid-induced acute diffuse lung injury and naturally occurring "nosocomial pneumonia." The bacterial cause of each pneumonia was established by either an immediate postmortem lung aspirate and/or antemortem blood culture. Bacterial pneumonia occurred in 12 of the 15 animals studied. The PSB cultures were sterile in 11 normal, intubated baboons and in 7 animals with non-pneumonic infiltrates by chest radiography. In each of these instances contamination of the specimen by proximal airway flora was avoided with the PSB. Among 10 baboons with bacteriologically documented pneumonias, the PSB cultures correctly identified the causative pathogen in 7 animals despite the presence of diffuse lung infiltrates radiographically, and multiple pathogenic bacteria in proximal airway secretions. Only 1 of 10 (10%) PSB specimens in these animals was contaminated with a possibly unrelated pathogen. The PSB largely avoided contamination of the culture specimen by proximal airway flora, and therefore should be useful in differentiating bacterial colonization of the airways from pneumonia in the presence of diffuse pulmonary infiltrates.