The presence of fibronectin on the surface of articular cartilage in rheumatoid arthritis (RA) was investigated. Cartilage samples were stained by the immunoperoxidase method using anti-human fibronectin antibody, and observed under light and electron microscopy. Fibronectin was present on the articular surface in 7 of 8 RA patients. The degree of staining varied greatly among the patients. Five of 8 patients were positive for fibronectin in 50% or more of the cartilage areas studied. In total, fibronectin was observed in RA. Fibronectin was not observed in cartilage samples of osteoarthritic joints or joints which were not diseased but had undergone trauma. Ultrastructurally, it was observed to be associated with collagen fibrils and amorphous substance in the matrix. The fibronectin-negative surface of the rheumatoid cartilage was usually thick ultrastructurally, compared with the fibronectin-positive surface, and the staining for fibronectin roughly correlated with decreased proteoglycans on the surface. The presence of fibronectin in the matrix appeared to be revealed by partial degradation of proteoglycans with proteolytic enzymes in the synovial fluid, as well as by the deposition of fibronectin onto the surface of rheumatoid cartilage. Fibronectin on the articular surface may play an important role in promoting pannus extension onto the articular surface in RA.