Wear particles released from synthetic anterior cruciate ligaments (ACLs) have been implicated as mediators of the effusions and synovitis that often follow ACL reconstruction. Particulate material, released as a result of abrasion and device failure, interacts with the synovial lining cells of the knee, causing inflammation, synovial hypertrophy, and cellular activation. This leads to the intraarticular release of degradative enzymes such as collagenase. By using collagenasespecific antiserum, the in vitro activation of synovial cells by small particles was observed immunofluorescently. This technique provided direct visual evidence of cellular activation as the result of the phagocytosis of particles of latex, carbon, or Dacron. Particles 23 μm and less in diameter were easily phagocytosed by the svnovial cells, resulting in the production of collagenase. This method permits observation of the interactions between individual cells and particles of specific sizes, shapes, and other physical properties. It should thus prove useful in future studies of the importance of these parameters in eliciting cellular responses to particles.