Zymosan particles, which are able to activate complement by the alternative pathway and to induce enzyme secretion from macrophages, were injected into knee joints of mice. After various time intervals, synovia were assessed histologically for various markers of inflammation. Within 7 days after intraarticular injection of zymosan, a chronic inflammatory arthritis with mononuclear cell infiltration, synovial hypertrophy and pannus formation was observed. Latex particles, which do not activate complement or macrophages, produced only mild, transient inflammatory reactions.