As part of a study of the characteristics of the synovial membrane which make it susceptible to inflammatory reactions, we tested a number of monoclonal antibodies (MoAb) which revealed novel features of the synovium using tissue from rheumatoid, osteoarthritic and traumatized (mechanically deranged) joints. In a previous study we detected macrophages (Mph) lining the synovial membrane by means of Mph specific and HLA-DR specific MoAb. These may account for the type A synoviocytes. Type B synoviocytes are thought to be fibroblastic and we used an anti-Thy 1 MoAb to identify these cells. Many fibroblasts were seen in a subintimal position but only few in the lining layer, not in sufficient numbers to account for the type-B category of synoviocyte. Staining with a new MoAb, 67, was found to precisely delineate the lining layer. This MoAb was previously seen to react with dendritic reticulum cells (DRC) of germinal centres, cells involved in B lymphocyte activation. However, other MoAb which react with germinal centre DRC did not label the synovial lining layer. Several MoAb revealed features of the vasculature not previously recognized. In rheumatoid samples, MoAb10 labelled small capillaries near the synovial surface and larger vessels deeper in the intima were labelled with a second MoAb, SM phi. This dichotomy of staining was not so apparent in synovium from control osteoarthritis and trauma (OA/T) samples. In addition, the Thy 1 epitope, identified previously on a variety of human cells, was strongly expressed on all vascular endothelium. Finally a new vessel or duct like structure was identified in OA/T samples, located subintimally. These ducts contained keratin, detected with MoAb LE61 and may be the normal counterpart for the rare malignancy, biphasic synovial sarcoma.