This study uses quantitative cultures and immunofluorescent demonstration of antibody-coated bacteria to differentiate colonizing from infecting bacteria in lower respiratory secretions obtained by fiberoptic bronchoscopy. The fiberoptic bronchoscope was passed transnasally without the use of a nasotracheal tube, and a single-sheath cytology brush dipped in lower respiratory secretions served as inoculum for quantitative cultures. Secretions were also collected by aspiration through the bronchoscope side channel for fluorescent examination. None of 60 control patients had greater than 4,000 colony-forming units (CFU) per brush of a single bacterium on quantitative culture. In contrast, more than 4,000 CFU per brush were isolated from 29 of 33 episodes of clinically defined lower respiratory infection (p less than 0.001). Only 1 of 60 control patients had antibody-coated bacteria in their lower respiratory secretions, but antibody coating was demonstrated in association with 24 of 33 episodes of infection (p less than 0.001).