Development of adjuvant disease in the rat is suppressed if hyperbaric oxygen is administered within one day after the inoculation of an oil suspension of Mycobacterium tuberculosis and continued for 16 to 17 days. Although 10 to 40% of the rodents developed mild arthritis after discontinuance of therapy, most remained symptom-free for at least 50 days. Oxygen administered after arthritis is advanced still exerted a significant curative effect. Possible mechanisms underlying the therapeutic action of oxygen are discussed.