v on Willebrand protein was found to promote the incorporation of platelets into evolving fibrin thrombi. Using formalin-treated or fresh platelets, both the initial rate and extent of platelet incorporation into polymerizing fibrin were dependent on von Willebrand protein. von Willebrand protein was incorporated into evolving fibrin thrombi in parallel with platelets. Soluble fibrin monomer covalently linked to acrylonitrile beads (Matrex 102) bound von Willebrand protein specifically and saturably with an apparent approximate dissociation constant (KD) of 15 micrograms/ml. Glycocalicin, the water-soluble proteolytic fragment of glycoprotein Ib, bound to fibrin monomer in this system specifically and saturably, as well, with an apparent approximate KD of 5 micrograms/ml, but only in the presence of saturating concentrations of von Willebrand protein. These data demonstrate that the initial rate and extent of platelet incorporation into evolving fibrin thrombi are dependent on von Willebrand protein; von Willebrand protein serves as a link between polymerizing fibrin and platelet surface glycoprotein Ib; and von Willebrand protein binds to fibrin monomer and is thereby able to bind to platelet surface glycoprotein Ib in the absence of ristocetin.