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By attaching native collagen and C1q to Sepharose, it was possible to test the binding of fibronectin (Fn) to the native and heat-denatured forms of these proteins without complications due to aggregation, precipitation, or fibril formation. Binding to the native proteins occurred only at low (sub-physiological) ionic strength whereas binding to the denatured proteins occurred even in I M NaC1. Thus both of these proteins possess one or more strong sites which are masked in the native state and become exposed during thermal denaturation. Fn did not bind to albumin-Sepharose or IgG-Sepharose either before or after heat-denaturations. C1q bound readily to native IgG-Sepharose but did not mediate the binding of Fn. Nor did Fn inhibit the reconstitution of C1 on antibody-coated erythrocytes. The fluorescence polarization of fluorescein-labelled collagen in 1 M NaCl displayed a downward transition at 38–40°C consistent with unfolding of the triple helix. In the presence of Fn, the same material displayed an upward transition at slightly lower temperature suggesting that gross unfolding is not required to expose the strong binding site(s). [ABSTRACT FROM AUTHOR]