The adherence of microorganisms to platelets previously immobilized on the subendocardium in nonbacterial thrombotic endocarditis is considered an important pathogenic step in Staphylococcus aureus endocarditis. To identify and characterize bacterial factors involved in the adherence to platelets, a phage display library of S. aureus was generated by use of the phagemid pG8H6. The library was affinity panned against purified immobilized platelets. After a second panning against platelets, a significant increase in the number of eluted phagemid particles was observed; 27% of 88 randomly isolated clones expressed overlapping deduced amino acid sequences with high similarity to the C-terminal domain of the S. aureus coagulase. In addition, 22% of the clones expressed the N-terminal domain of the fibrinogen-binding protein Efb. The surface-associated fraction of the C-terminal domain of coagulase or the N-terminal domain of Efb may be involved in bacterial adherence to immobilized platelets, and fibrinogen may act as a bridging molecule in that interaction