Platelets adhering to artificial or biologic surfaces have been implicated in the pathogenesis of catheter infections or endocarditis; however, the ligands involved in Staphylococcus aureus interaction with adherent platelets remain incompletely understood. Radiolabeled S. aureus Cowan I were incubated with purified platelets adherent to polymethylmethacrylate (PMMA) coverslips and washed, and adhesion was determined. Platelets promoted adhesion of S. aureus approximately 30-fold compared with adhesion to albumin-PMMA. In the presence of both plasma (1% vol/vol) and platelets, adhesion was extensively promoted, with 30% (of inoculated) S. aureus adherent (150-fold increase). Platelet pretreatment with anti-GPIIb/IIIa monoclonal antibodies or inhibitors of platelet activation decreased plasma-enhanced adhesion, suggesting a role of platelet activation in S. aureus adhesion. Plasma-enhanced adhesion was sensitive to thrombin antagonists, proteinase inhibitors, heparin, or antifibrinogen antibodies, indicating that fibrinogen/fibrin is necessary for bridging between adherent platelets and S. aureus. In conclusion, S. aureus adhesion to immobilized platelets may play a role in the pathogenesis of invasive bloodstream infections or endocarditis.