The production of interleukin 1 (IL1) by peritoneal and splenic macrophages from rats with adjuvant-induced arthritis on day 17 postadjuvant treatment was not altered compared with normal. Treatment of arthritic rats with beta-carotene reduced hind paw swelling and significantly increased ability of macrophages to secrete IL1 as well as stimulated spontaneous proliferation of splenic lymphocytes. No direct relationship between the release of IL1 from peritoneal and splenic macrophages and increase of hind paw swelling was revealed.