The effect of purified von Willebrand Factor (vWF) fragments, SpII (dimer of two 110 kd subunits) and SpIII (dimer of two 170 kd subunits) obtained with S aureus V-8 protease was tested upon platelet adhesion to collagen. Purified fibrillar human collagen coated onto cover slips was incubated with SpII, SpIII, or undigested vWF and exposed to reconstituted human blood in a parallel-plate perfusion chamber at a high shear rate. Platelet-collagen interactions were estimated using 51Cr-platelets and quantitative morphometry. When blood was reconstituted with citrated autologous plasma, SpIII and vWF strikingly enhanced platelet adhesion to collagen whereas SpII had no effect. When blood was reconstituted with human albumin and divalent cations, SpIII and vWF again promoted platelet adhesion to collagen. In conclusion, our data suggest that (1) SpIII, the N-terminal portion of vWF which binds to platelet membrane glycoprotein Ib, functionally substitutes for vWF in supporting platelet adhesion to collagen; (2) SpII, the C- terminal portion which binds to glycoprotein IIb/IIIa, has no such effect; (3) in addition to its platelet binding domain, SpIII contains another site for binding to collagen; and (4) the multimeric structure of vWF is not required for platelet adhesion to collagen.