Fibronectin is an extracellular matrix protein that is important in development, wound healing and tumorigenesis. In the blood it is dimeric, but in tissues forms disulphide crosslinked fibrils. Here we show that a fragment from the first type-III repeat of fibronectin binds to fibronectin and induces spontaneous disulphide crosslinking of the molecule into multimers of high relative molecular mass which resemble matrix fibrils. Treatment of fibronectin with this inducing fragment also converts fibronectin into a form that has greatly enhanced adhesive properties (hence the term superfibronectin) and which suppresses cell migration. Whereas cells attach to fibronectin through integrins, cell attachment to superfibronectin is mediated both by integrins and by receptors with properties distinct from those of integrins. Superfibronectin may be closely related to the natural matrix form of fibronectin.