The bacterial flora of non-inflamed maxillary sinuses were studied in 12 adults. Patients' median age was 35 years (range 25 to 53 years): 8 were females and 4 were males. Aseptic aspiration of the sinus was made when the patients were under general anaesthesia for corrective plastic surgery of the nasal septum. All aspirates were cultured for aerobes and anaerobes. Anaerobes were isolated in all 12 specimen patients; in 5 patients (42%), they were the only organisms isolated, and in 7 (58%), they were recovered mixed with aerobes. There were 33 anaerobic isolates (2.75 pr specimen) and the predominant ones were: 11 Bacteroides sp. (including 6 B. melaninogenicus), 10 anaerobic gram-positive cocci and 5 Fusobacterium sp. There were 16 aerobic isolates (1.3 per specimen) and the predominant ones were: 5 beta hemolytic streptococci, 3 alpha hemolytic streptococci, and 2 each of S. pneumoniae and hemophilus parainfluenzae. Beta lactamase production was noted in 4 isolates recovered from 3 patients. These included all isolates of S. aureus (2) and 2 of 6 isolates of B. melaninogenicus. The presence of aerobic and anaerobic organisms in the non-inflamed sinus demonstrates the non-sterility of the sinus cavity. In the events that follow closure of the sinus ostium, these bacteria may become pathogenic.