The radius of gyration of human plasma fibronectin was determined by light scattering both under conditions in which the molecule is in an extended conformation (ionic strength 1.01 M, pH 8) and close to its native, more compact conformation (ionic strength 0.16 M, pH 8). These values were found to be 17.5 +/‐ 0.8 nm and 10.7 +/‐ 0.9 nm respectively, for a constant mol. wt of 533,000 +/‐ 8000, in excellent agreement with the value of 520,000 deduced from its known composition. A set of models, each made of two identical, end‐to‐end joined chains of 28 beads, was then constructed, and their calculated physico‐chemical parameters were compared with those available for the whole fibronectin molecule and for some of its proteolytic fragments in both conformations. Two possible models for the circulating form are presented here: in both, the fibronectin molecule is in a compact, tangled conformation, with the amino‐terminal end of one chain folded over to the carboxy end of itself or of the other chain either in a hairpin or in a circular fashion. With the exception of the carboxy‐terminal fibrin(ogen)‐binding domains, all the domains appear to be well exposed to the solvent, and thus free to interact with potential ligands.