A collagen-binding domain of von Willebrand factor (vWF) has been identified in the central part of the molecule by comparing the binding properties of vWF and Staphylococcus aureus V-8 protease-generated vWF fragments with collagen. The binding of purified human vWF to human type III collagen was found to be specific. At saturation, 38 to 50.2 micrograms of vWF bound per milligram of collagen. Scatchard plots derived from binding isotherms demonstrated the presence of at least two classes of binding sites. Purified vWF was digested with S aureus V-8 protease into two complementary fragments (SpIII and SpII). SpII, the C-terminal end of vWF (amino acid residues 1,366 to 2,050), was totally devoid of affinity for collagen. Contrarily, purified SpIII, the N-terminal part of vWF (residues 1 to 1,365), totally displaced vWF binding and specifically bound to collagen. At saturation, 25 to 45 micrograms of SpIII bound per milligram of collagen. Scatchard plots demonstrated the presence of a single class of binding sites. SpIII was further digested with the same enzyme to generate SpI, a 52-kilodalton fragment from the C-terminal part of SpIII (residues 911 to 1,365). Spl induced a dose-dependent inhibition of both vWF and SpIII binding to collagen. A series of six monoclonal antibodies against SpIII that completely abolished vWF and SpIII interaction with collagen also bound to SpI. In conclusion, SpI extending between amino acid residues 911 and 1,365 of vWF contains a specific site that interacts with human type III collagen.