A quantitative investigation on the effects of plasma proteins and platelets on the adhesion of Staphylococcus epidermidis RP62A to a hydrophobic biomedical polymer (National Heart, Lung, and Blood Institute reference polyethylene) was carried out under well-defined shear conditions approximating human blood circulation by using a rotating disk system. The results showed that contact-activated platelets mediated S. epidermidis adhesion to the polymer surface. In the range of physiologic shear conditions, the adhesive coefficient (ratio of bacteria per unit area to the product of bacterial flux and the duration of the experiment) to platelets was significantly greater than to the protein-adsorbed polyethylene surface by at least one order of magnitude (P < or = .01). The presence of absorbed plasma proteins on polyethylene reduced the adhesion of S. epidermidis compared with that seen with the bare polymer surface. These studies show that S. epidermidis adhesion to polyethylene is mediated by contact-activated platelets, not absorbed plasma proteins.