To investigate the morphology of the synovial lining cells, synovitis was induced by carrageenin injection into the rat knee joint cavities. Synovial membranes were excised at 1, 3, 5, 7 and 14 days, and histologic, electron microscopic, histochemical (periodic acid Schiff: PAS, toluidine blue), and enzyme histochemical (acid phosphatase: ACP, nonspecific esterase: NSE and endogenous peroxidase) studies were performed. The results are as follows: Carrageenin induced synovial membrane hypertrophy with synovial cell proliferation and granuloma formation. The proliferated synovial lining cells and macrophages in the granulomatous lesion had round to oval nuclei and large, plump cytoplasm with many phagocytotic materials and vacuoles. Electron microscopically, these cells had small number of granular endoplasmic reticulum and many lysosomes, phagosomes and vaculoes. Mitotic figures were observed at early stage of experiment. PAS and toluidine blue stains showed strongly positive reaction in the cytoplasm of the proliferated lining cells and macrophages in granulomatous lesion. ACP and NSE activities were strong positive in the cytoplasm of the proliferated synovial lining cells and macrophages in the granulomatous lesion. But endogenous peroxidase stains were negative in all prolifeative lining cells and macrophages in granulomatous lesion. Conclusively, carrageenin-induced synovitis showed proliferation of synovial lining cells and granuloma formation in deep layer. The macrophages, which consisted of the lesions and have active phagocytic function, were speculated to proliferate by mitosis of superficial synovial A cells and histiocytes in the deep layer of the synovial membrane.