The role of subendothelial fibronectin in platelet interaction with subendothelium was studied. Human umbilical artery subendothelium was exposed to flowing blood containing 111In-labeled platelets in an annular perfusion chamber. Platelet adhesion was determined from the 111In radioactivity on the vessel wall. When perfusions were performed for five minutes at a wall shear rate of 1,800 s-1, platelet adhesion was the same whether normal plasma or fibronectin-free plasma was used. Preincubation of subendothelium with rabbit anti-human fibronectin serum, however, resulted in a marked inhibition of platelet adhesion. Preincubation with normal rabbit serum had no effect. Platelet adhesion was also diminished when the vessel wall was preincubated with anti- fibronectin IgG fraction or F(ab')2 fragment. After the latter preincubations, frozen sections of 4 micron were incubated with fluorescein isothiocyanate-conjugated goat anti-rabbit IgG, F(ab')2 fragment specific. Fluorescence was seen throughout the subendothelium both before and after perfusion. No fluorescence was seen when subendothelium was preincubated with normal rabbit IgG or F(ab')2 or with anti-fibronectin IgG that had been absorbed with purified fibronectin. After absorption of anti-fibronectin IgG with purified fibronectin, the inhibiting effect on platelet adhesion was also no longer present. Preincubation of the vessel wall with anti-fibronectin IgG reduced platelet adhesion significantly at a wall shear rate of 800 s-1. This effect was even greater at 1,800 s-1. At low shear rate (400 s-1), there was no inhibition.