Fibronectin remains unique among matrix components because it exists in both soluble and matrix forms. It seems likely that the necessity to prevent inappropriate matrix assembly from soluble fibronectin results in a strict requirement for catalysis by specific cells to deposit fibronectin fibrils. Moreover, many cell types possess adhesive receptors for fibronectin (Buck & Horwitz 1987), but only certain mesenchymal cells appear capable of depositing organized matrices. These professional matrix-organizing cells somehow interact with the aminoterminus of fibronectin and construct fibrils, preventing diffusion of fibronectin away from sites of synthesis and forming an extracellular matrix with a very high concentration of fibronectin. The resulting matrix should facilitate the attachment and migration of macrophages and neural crest cells that possess adhesive receptors but lack matrix-forming ability. Thus, it is possible that this system of dual cell interactive cells evolved in order to allow careful modulation of cell interactions with fibronectin leading primarily to synthesis and deposition (fibroblasts), or to cell recognition (neural crest, macrophages). Elucidation of the cell surface molecules interacting with fibronectin's aminoterminus is a critical first step in further understanding this adhesive and matrix assembly system.