The quantitative analysis of circular dichroic spectra of native human plasma fibronectin according to the method of Provencher and Glöckner [Provencher, S. W., & Glöckner, J. (1981) Biochemistry 20, 33-37] indicated the presence of beta-sheet (79%), beta-turn (21%), but no alpha-helix or random coil in the secondary structure. The calf alveolar heparan sulfates induced a change in the conformation of fibronectin: the magnitude of the change depended on the molecular properties of the particular heparan sulfate preparations.