The nature of the supramolecular complex between fibrillar collagen and collagen-binding proteins (CBPs) has hindered detailed X-ray and NMR analyses of the ligand-recognition mechanism at atomic resolution because of the lack of appropriate approaches for studying large heterogeneous supramolecular complexes. Recently, we proposed an NMR method, termed transferred cross-saturation (TCS), that enables the rigorous identification of contact residues in a huge protein complex. Here we used TCS to study the supramolecular complex between the A3 domain of von Willebrand factor and fibrillar collagen, which allowed the successful determination of the ligand-binding site of the A3 domain. The binding site of the A3 domain was located at its hydrophobic 'front' surface and was completely different from that of the I domain from the a2 subunit of integrin (alpha2-I domain), which was reported to be the hydrophilic 'top' surface of alpha2-I, although the A3 domain and the alpha2-I domain share a similar fold and possess the identical function of collagen binding.