Von Willebrand factor is adsorbed from plasma by fibrillar collagen in a manner which is dependent upon the time of incubation and collagen concentration. The adsorption does not require divalent cations and is temperature independent. As purified von Willebrand factor is also adsorbed by fibrillar collagen it is unlikely that the adsorption is mediated by other plasma proteins. Denatured collagen has no effect on von Willebrand factor activity and does not inherit the adsorption of the factor by native fibrillar collagen. The adsorbed von Willebrand factor can be eluted from fibrillar collagen with 1M NaCl. The similarities between the adhesion of platelets to collagen and the adsorption of von Willebrand factor by collagen suggest that von Willebrand factor may have a role in collagen-platelet adhesion. The observed inhibition of platelet adhesion to collagen by antiserum to von Willebrand factor is consistent with this hypothesis.