In a preliminary publication1 I have shown the presence of pathogenic bacteria (predominately Staphylococcus albus and Staphylococcus aureus) in the air of the operating rooms of the Duke Hospital. Various regulations for reducing the numbers of these bacteria were described, the most important of which follow: 1. Rigid isolation was practiced. No visitors were permitted; the personnel of the operating room was kept as small as possible, and no one was allowed to enter the rooms when they were not in use. 2. Large, heavy masks were worn over the nose and mouth by all occupants at all times, regardless of whether an operation was in progress. 3. All persistent carriers of Staph. aureus in the nose and throat were excluded, and important operative procedures were postponed when the contamination of the air was high. 4. Scrupulous cleanliness was maintained by frequent washing and painting and by eliminating air