Despite studies of the pathologic and microbiologic aspects of the lung In cystic fibrosis (CF), there is a lack of information on the lung localization of bacterial pathogens. Bacteriologic data come from cultures of sputum or accessible lung effluent from bronchoscopy. Our objective was to localize Pseudomonas aeruginosa (PA) in situ In order to provide descriptive data on the relationship between the presence of PAand disease of the surrounding tissue. Using stored, uncut blocks of preserved lung, we deparaffinized, cut, mounted, and reacted them with high-titer rabbit sera made to a mucoid strain of PA. The tissues were then reacted with a second antibody, biotinylated goat antirabbit Immunoglobulin, developed using a peroxidase technique and counterstalned with hematoxylin. PAorganisms stained with heavy brown deposits and this was species-specific.