Fibronectin is an adhesive glycoprotein synthesized by mesenchymal cells. Its distribution in the rheumatoid pannus has been studied by immunofluorescence using a monospecific antiserum. All areas of the pannus contained immunoreactive fibronectin, including its junctions with synovium, ligaments, bone and cartilage. It formed a coarse extracellular meshwork which surrounded the inflammatory cells infiltrating the pannus and which codistributed with reticulin and" immature" collagen. The proliferative cellular areas of the pannus showed marked reactivity for fibronectin. Although fibronectin was present at the pannus-cartilage junction, it was not otherwise found in articular cartilage. Fibronectin in the pannus did not have any relationship to the distribution of immunoglobulins or complement. Fibronectin is a structural component of the rheumatoid pannus and may play a role in the pathogenesis of rheumatoid synovitis.