The distribution of fibronectin in the inflamed synovium has been described previously using immunohistochemical methods. Under favourable conditions, it is possible to demonstrate apparent cytoplasmic staining of the intimal cell layer. We have further investigated the localisation of fibronectin in the synovial intimal cells using higher resolution techniques with peroxidase-antiperoxidase staining and high power light microscopy of semi-thin Araldite sections and immunoelectron microscopy using a protein A-gold technique. Synovia from 11 mechanical/traumatic, or osteoarthritic joints; 12 seropositive rheumatoid arthritis and nine cases from other joint diseases made a total of 32 cases examined in semi-thin sections, while six rheumatoid and two osteoarthritis synovia were studied by immunoelectron microscopy. Fibronectin was demonstrated in individual cells of the synovial intimal layer in 22 out of 32 samples examined by the light microscope method, and electron microscopy of adjacent sections showed that the positively staining cells were type B synoviocytes. Immunoelectron microscopy confirmed the presence of fibronectin within the rough endoplasmic reticulum of type B synoviocytes in all but one of the eight samples examined. The results provide evidence that the type B synoviocyte is responsible for fibronectin production.