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Human plasma fibronectin and a series of its large proteolytic fragments were analyzed by electron microscopy using tungsten shadowing on carbon and polystyrene films. On carbon, intact fibronectin appeared as a randomly coiled strand, while on polystyrene it appeared as an elongated structure. Two fragments of fibronectin, M, = 205000 and 190000, which lack the NH2-terminal domain of fibronectin and retain the collagen-binding, cellattachment and heparin-binding functions, and a M , = 170000 fragment, which retains the collagen-binding and cell-attachment functions, were seen as rods with varying degrees of nodularity while a M , = 100000 fragment, which only binds to collagen, had two clear-cut domains.These results support the existing biochemical evidence that the segregation of the functional activities in the fibronectin molecule is based on distinct structural domains and provides evidence for the existence of an additional structural domain not revealed by biochemical and functional studies.

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