The reliability of fiberoptic bronchoscopy as a method to study the bacteriology of the lower respiratory tract was tested. The procedure used was suction aspiration through the inner channel after topical anesthesia with lidocaine. To detect contamination by oropharyngeal bacteria, the aspirates were cultured in patients with no evidence of active infection, comparison was made with results of transtracheal aspiration cultures, and the aspirate was tested for the presence of an oral dye marker. Results with all 3 methods of analysis indicated contamination with oropharyngeal bacteria that were presumably introduced during instrumentation through the upper airways. An additional factor studied was the effect of topical anesthetics. Analysis of aspirates showed that as much as 96 per cent of the specimen was anesthetic solution. Lidocaine also proved toxic to lower respiratory tract pathogens, although there were significant differences between bacterial species. It was concluded that fiberoptic bronchoscopy as performed in this study does not reliably reflect the bacteriology of the lower respiratory tract.