Patients who had upper respiratory tract gram-negative bacillary colonization and noncolonized patients were followed through surgical procedures to determine what bacterial organisms would be deposited in anesthesia apparatus. Anesthesia machines were cultured for bacteria in many locations before and after each surgical procedure. Six machines in routine operating room use were studied after use on six colonized patients and nine uncolonized patients. Sixteen corrugated tubes from unopened packages served as controls. The results indicated that the machines remained free of bacteria of patient origin. Levels of contamination were only slightly higher in the expiratory tubing, and the bacterial species most commonly recovered were environmental in origin. Even after periods of anesthetic administration as long as six hours in patients heavily colonized with gram-negative bacilli, contamination of the anesthesia apparatus with the colonizing organisms did not occur. Intentional contamination of a sterilized anesthesia machine with two gram-negative organisms confirmed the clinical observations. Analysis of oxygen and nitrous oxide gas sources for bacteria had negative results. Basic hygienic management of anesthesia machines will ensure safety from the standpoint of cross-infection.