Fibronectin is produced by several cell types, with hepatocytes currently recognized as the main source of plasma fibronectin. Mesenchymal cells produce fibronectin in vitro and probably do so even in vivo. But it is not clear if these cells represent the sole source of tissue fibronectin. In the present study, using direct and indirect immunofluorescence, fibronectin was found to occur ubiquitously in connective tissue in the rat. When 125I-labelled fibronectin was intravenously injected in rat, there was significant uptake of fibronectin in the tissues. The half-time for removal of labelled fibronectin in circulation was c. 15 hours. The authors conclude that flux of fibronectin occurs from plasma into the tissues and that in normal circumstances plasma fibronectin contributes to fibronectin in tissues.