Rats with adjuvant arthritis were treated with estrone, 1.0 mg/rat twice per week, beginning 10 days before the adjuvant injection. When the hormonal treatment was continued for 5 weeks, almost complete inhibition of the disease was observed. When the treatment was terminated before the first signs of the disease were observed or before the administration of the adjuvant injection, a slight suppression of the disease was found. Adjuvant arthritis was also inhibited by estrone when the hormonal treatment was begun after the disease had become macroscopically manifest. An estrone dose of 0.25 mg/rat twice a week was sufficient to produce suppression. A 10 mg/rat dose of progesterone twice a week had no definite influence on the disease. In microscopic examination, non-caseous tubercle-like granulomata were observed in the joints, lungs and the liver, but not in the spleen. In the joints, they occurred most often in the presence of severe exudative lesions. In the lungs and liver the granulomata were without any other inflammatory alterations, and in the rats treated with an estrone dose of 1.0 mg, they were found to a lesser extent than in the other groups.