The reduction of inflammatory reaction by cyclophosphamide administration appears to be caused mainly by selective depletion of the neutrophils, and partly by immune suppression. It appears that the rabbit abdominal macrophage has the common morphologic, functional and antigenic patterns with the M-type synovial lining cells. There is another possibility that the cross-reacting antigens between macrophage and the M-type cell of the synovial lining may exist. It is concluded that in this experimental immune complex arthritis, the site of localization of immune complexes seems to be the synovial, M-type cell, and the tissue injury of synovium is largely mediated not only by neutrophils and complement, but also by macrophages.