ƒÞ S. aureus infection begins when bacterial cells circulating in blood adhere to components of the extracellular matrix or endothelial cells of the host and initiate colonization. S. aureus binding to activated platelets may represent a key step in spreading of infection. Prior work has shown that clumping factor A of S. aureus is one of the major surface proteins mediating the bacterial adhesion to platelet via fibrinogen (Fg). On the contrary, fibronectin binding proteins(FnBPs) of S. aureus has been known to contribute to the bacterial penetration to the matrix. Here we investigate how FnBP of S. aureus is involved in the bacterial adhesion to platelets in shear flow. At shear rate 2000s-1, cells of the early growth phase of S. aureus Phillips exhibited high levels of adhesion to platelets that is twice as high as the adhesion mediated by the clumping factor A of S. aureus Newman strain. Both Fg and fibronectin could support the binding suggesting that the protein is the FnBP of S. aureus. Antagonist XV454 which blocks the binding of Fg to GPIIb/IIIa of platelet showed an extensive inhibition against bacterial adhesion to platelets in the presence of Fg while the inhibition is less effective for the fibronectin-mediated binding. Monoclonal antibody against clumping factor A of S. aureus did not inhibit the observed adhesion. The adhesion is shear- dependent with highest adhesion at the shear rate of 2000s-1. Purification of bacterial lysate using both Fg-affinity chromatography and gel chromatography produced a protein which was identified as FnBP. In the presence of the purified protein, the binding of Fg to the bacteria and the adhesion of bacteria to the platelets were significantly inhibited. In this presentation, the possible explanations for the extraordinary binding of S. aureus Phillips strain to the platelets by FnBP will be discussed.