The presence of cells bearing epitopes of the mononuclear phagocyte series was studied immunohistochemically in synovium removed from joints involved by trauma (T), osteoarthritis (OA) and rheumatoid arthritis (RA). Mononuclear phagocytes were the most consistent feature of the inflamed rheumatoid synovium. They shared at least five epitopes expressed by mononuclear phagocytes in other tissues. In OA/T samples, cells bearing markers of the less mature monocyte were present at the surface of the synovial membrane, namely the intimal layer, while those bearing macrophage epitopes were apparent throughout the intimal layer and subintimally. This suggested that maturation of the monocyte population takes place after the monocytes have entered the synovial tissues, settled at the surface, then moved downward into the subintimal layer. The synovial monocytes accounted for all the HLA-D region positive cells in the lining layer.