The aerobic fecal and oropharyngeal bacterial flora was examined in 75 patients hospitalized, but not given antibiotics; in 70 patients given antibiotics during hospitalization and in 25 nonhospitalized controls. In all subjects, when first examined, normal throat flora were predominant. At the end of 21 days, however, a gram-negative bacilli became predominant in 17 (22.7%) of the Hospital Group and 33 (47.1%) of the Antibiotic Group. Newly appearing genera of gram-negative bacilli in the pharynx were almost always represented those present in the fecal flora. The Hospital Group all had recognizable components of the normal oropharyngeal flora present at 21 days, but 12 (17.1%) of the Antibiotic Group had no demonstrable normal oropharyngeal flora at 21 days. The findings suggest that hospitalization alone can be associated with the appearance of gram-negative bacilli in the oropharynx, and that the intestinal tract is their most likely point of origin.