The incidence of rice bodies (RB) in synovial effusions has been studied in 36 patients with rheumatoid arthritis (RA) and in 12 patients with seronegative inflammatory arthritis (7 cases of Still's disease, 3 of psoriatic arthritis, and 2 of ankylosing spondylitis). In the RA group 50 joints were aspirated before and after saline lavage with a specially designed wide-bore needle. RB were found in 72% overall of the joints studied in this group, 14% on initial simple aspiration and an additional 58% after lavage. In contrast no rice bodies were found in 31 aspirations with lavage by an identical technique in the 12 patients with seronegative synovitis. The RB in RA synovitis occurred both early and late in the course of the disease and were not related to the severity of clinical or radiological changes. However, removal of rice bodies was accompanied by clinical improvement and reduction of synovitis. Macroscopically RB varied in shape and size, some being so large as to preclude effective removal by needles of the gauge customarily employed for joint aspirations. Microscopically the majority of RB were composed of coarsely reticular material reacting immunologically with antifibrinogen and antifibronectin and containing mononuclear cells. Some showed vacuolation suggestive of fibrinolysis, but many showed organisation like that seen in established connective tissues, with the formation of mature collagen, reticulin, and elastin. These findings are discussed in relation to the origin, development, and significance of rice bodies in rheumatoid synovitis.