In this study we investigated mechanisms involved in the chronic character of experimental collagen type II induced arthritis (CIA). We compared the knee joints of mouse strains which are prone to develop this autoimmune disease (DBA/1,B10RIII) with other nonsusceptible mouse strains (C57Bl/6,BALB/c) in their reaction to different stimuli: immune complexes (IC), zymosan and streptococcal cell walls (SCW). Inflammation was evaluated by^99mTc uptake measurements and in haematoxylin- and eosin-stained knee-joint sections. Passively induced immune complex mediated arthritis (ICA) in knee joints of C57Bl/6 and BALB/c mice, showed moderate cell influx at day 3, whereas at day 7 only minor amounts of inflammatory cells were observed. In contrast, in arthritic DBA/1 and, to a lesser extent, in B10.RIII joints, a tremendous cell influx was observed at day 3 and even at day 14 there was still significant synovitis. In contrast, if arthritis was elicited by intra-articular injection of zymosan or SCW in C57Bl/6 and DBA/1, the course of inflammation was similar in both strains and no chronic inflammation developed. In line with severe arthritis, chemotactic factor production was dramatically enhanced in ICA in DBA/1 mice, and a prolonged production of IL-1 was evident. When IL-1 was neutralized before or during the ICA using specific anti-IL-1α,β antibodies, inflammation could be blocked completely. Single or multiple injection of IL-1 in the knee joint of C57Bl/6 or DBA/1 showed comparable inflammation, indicating that the chemotactic response per se is comparable in both strains. No prolonged production of IL-1 was found during zymosan or SCW arthritis. Selective removal of macrophages from the synovial intima prior to ICA induction (using clodronate-containing liposomes) prevented the onset of inflammation in C57Bl/6 and DBA/1 mice. It can be concluded that immune complexes, but not zymosan or SCW, cause a more severe and chronic arthritis in mouse strains which are susceptible for collagen type II autoimmune arthritis. This is due to higher and prolonged expression of IL-1 and chemotactic factors, caused by stimulation with immune complexes. The interaction of IC with lining macrophages probably plays a dominant role in development of chronicity.