Platelet deposition at high wall shear rates on collagen type I and type III purified from human umbilical arteries is dependent on the presence of fibronectin and von Willebrand factor (VWF). The role of fibronectin at low wall shear rates (less than or equal to 500 s-1), where platelet deposition was independent of VWF, was studied with purified collagen I and III. Platelet deposition on nonfibrillar collagen I was fibronectin-dependent at all wall shear rates. Platelet deposition on nonfibrillar collagen type III was fibronectin-dependent at 300 s-1 and higher shear rates. By using a mixture of nonfibrillar type I and III, platelet deposition was found to be fibronectin-dependent at the tested wall shear rates (20, 100, and 300 s-1). This dependency was less than with nonfibrillar type I only, but more than with nonfibrillar type III. For platelet deposition on reconstituted type I or type III collagen fibrils, no fibronectin dependency was observed up to the highest wall shear rate tested (1800 s-1). The same results were obtained with a mixture of native type I and III fibrils. Thus, the dependence of platelet deposition on fibronectin is determined by the collagen type and the wall shear rate. The dependence on the fibronectin concentration was tested with nonfibrillar collagen type I at a wall shear rate of 300 s-1. Platelet deposition increased with increased fibronectin concentration up to a level of 700 micrograms/ml and leveled off above this concentration.