Samples of expectorated sputum were examined grossly and microscopically to determine their suitability for bacterial cultures. Microscopically, specimens were categorized according to the number of leukocytes and squamous epithelial cells (SEC) observed under low-power (times 100) in a Gram-stained smear. The mean number of species isolated was greater than 4 from specimens with more than 10 SEC per field, 2.7 from specimens with fewer than 10 SEC per field, and 2.4 from transtracheal aspirates. Oropharyngeal flora was isolated from nearly all of the specimens with more than 10 SEC per field, and potential pathogens were found in less than 15% of such specimens. The bacterial flora of specimens with fewer than 10 SEC per field closely resembled that of transtracheal aspirates.