In the cartilage-pannus junction of 14 patients with rheumatoid arthritis (RA) and seven patients with osteoarthritis (OA), monoclonal antibodies to keratan sulphate (KS) and chondroitin sulphate (CS) stained a transitional fibroblastic zone (TFZ) within the pannus in nine RA patients and one OA patient. In three patients this was clearly localised to the cytoplasm of cells in this zone, but in all remaining cases KS and CS could be demonstrated in the surrounding matrix. This area was distinguished from adjacent pannus which contained many blood vessels and cells positive for MHC Class II antigen. Specific markers for glycosaminoglycans have been employed to demonstrate that chondrocyte-derived cells and matrix contribute to the changes seen at the cartilage-pannus junction in RA-affected joints.