The hypothesis that normal lung is sterile was examined. A laboratory animal, the dog, was chosen so that sample size could be maximized. Controls were used to eliminate the possible artifactual presence of bacteria in the lung. Thirty-seven percent of lung samples contained aerobic bacteria with a mean concentration of 1.3 .times. 103 organisms/g tissue. Seventy-four percent of identical bacterial isolates were found in the pharynx of the same animal. Viable bacteria are apparently aspirated into normal lung where they may survive for at least limited intervals.