The contributions of clumping factor and coagulase in mediating Staphylococcus aureus adhesion to surface-adsorbed fibrinogen have been quantified by using a new methodology and analysis. The attachment or detachment kinetics of bacteria were directly observed in a radial flow chamber with a well-defined laminar flow field and a spatially varying shear rate and were quantified by recursively scanning the chamber surface and counting cells via automated video microscopy and image analysis with a motorized stage and focus control. Intrinsic rate constants for attachment or detachment were estimated as functions of shear rate for the wild-type Newman strain of S. aureus and for mutants lacking clumping factor, coagulase, or both proteins on surfaces coated with plasma, fibrinogen, or albumin. Clumping factor, but not coagulase, increased the probability of attachment and decreased the probability of detachment of S. aureus on plasma-coated surfaces; however, both clumping factor and, to a lesser extent, coagulase increased the probability of attachment on the purified-fibrinogen-coated surface. All mutants were resistant to detachment on the purified-fibrinogen-coated surface, suggesting the possibility of an additional adhesion mechanism which was independent of coagulase or clumping factor and effective only for fully attached cells. Together, these results suggest that the presence of clumping factor plays the primary role in enhancing adhesion to surfaces with adsorbed fibrinogen, not only by enhancing the probability of cell attachment but also by increasing the strength of the resulting adhesion.