The relative importance of oxygen in relation to resistance to infection was assessed in 24 mongrel dogs. Rectus abdominis musculocutaneous (MC) and corresponding cutaneous random pattern (RP) flaps based at the level of the xiphoid were elevated, replaced, and sutured. Immediately after the surgical procedure, 0.1 ml saline containing 10(8) Staphylococcus aureus/ml was injected intradermally into six fields of each flap. After the operation, the dogs were caged in controlled environments with different oxygen concentrations at 12%, 21%, and 45% for 3 days. After 24, 48, and 72 hours, lesion size was measured. Different lesion size was noted between the hypoxic and the hyperoxic groups in the MC flaps from the first day on and in the RP flaps from the second day on (p less than 0.05). Resistance to infection with S. aureus is oxygen dependent, particularly when tissue PO2 is below 40 mmHg.