Synovial retention of intra-articularly administered antigen was studied in rabbits with immune arthritis. Synovitis was induced in 6 rabbits by the intra-articular injection of ovalbumin following systemic immunization with this antigen. One month following induction of immune arthritis, synovial fluid macrophages were collected, washed and incubated with radioisotope labelled rabbit IgG antibody to ovalbumin or with labelled normal rabbit IgG (devoid of anti-ovalbumin activity). Autoradiographic studies revealed that 1·5 to 2·5% of the synovial macrophages were labelled by the anti-ovalbumin antibody; no labelling was seen when the cells were incubated with labelled normal rabbit IgG. These data indicate that at least a portion of the antigen retained by the rabbit synovium is bound to or associated with synovial macrophages.