A relationship between the production of interleukin 1 (IL-1) by macrophages from adjuvant-induced arthritic rats and cytochrome P-450-dependent hepatic microsomal monooxygenase was studied. The synthesis of IL-1 by splenic and peritoneal macrophages on day 17 postadjuvant treatment was not altered, but the hepatic cytochrome P-450 levels and monooxygenase activity were significantly decreased. Beta-carotene treatment of arthritic rats reduced hind paw swelling and concurrently stimulated the ability of macrophages to secrete IL-1 and increased the cytochrome P-450 levels and the activity of hepatic monooxygenase. The findings did not establish a definite relationship between the production of IL-1 by systemic macrophages on the one hand, and the hepatic cytochrome P-450 levels a and monooxygenase activity on the other hand. It thus appears that IL1 is unable to play a role of a mediator between the immune system and the hepatic cytochrome P-450-dependent monooxygenase system of rats with adjuvant-induced arthritis.