Although fibronectin's role as a matrix to improve endothelial seeding has been demonstrated by other workers, the optimum concentration for use has never been described. Attachment of fibronectin to expanded polytetrafluoroethylene (ePTFE) was measured, using 125I‐radiolabeled protein, at different concentrations and for different time periods. The absolute amount of fibronectin bound to the graft increased with the concentrations used in coating (p < 0.001) and also with time (p < 0.01); e.g., at 50 μg/ml, 90 min of incubation produced a molecular attachment of 4.0 × 1011/cm2 of graft. However, its percentage attachment decreased with a rise in concentration (p < 0.001). After an initial loss of 22% in 30 min, the fibronectin‐graft bond was found to be stable when exposed to a shear stress produced by flow at 200 ml/min. No significant difference in the cell adherence could be found in grafts coated with fibronectin concentrations of 50, 150, and 250 μg/ml, although it was significantly less at 10 and 25 μg/ml (p < 0.05).