The ability of Staphylococcus aureus Cowan I strain and a number of S. aureus clinical isolates to bind to the human blood glycoprotein clusterin was investigated. Binding of clusterin to these strains was tested by both enzyme-linked immunosorbent assay and flow cytometry. All of the S. aureus strains examined appeared to bind clusterin to some extent, while nonpathogenic control strains Bacillus subtilis BR151 and Escherichia coli JM109 did not. Three S. aureus isolates were selected for more detailed study; binding of labeled clusterin was saturable, inhibited in the presence of excess unlabeled clusterin, and prevented by pretreatment of bacteria with proteases. From the saturation binding studies, estimates of the affinity constants for the binding o clusterin to the bacteria ranged from 31 to 57 nM. Addition of clusterin to S. aureus cultures was also found to result in aggregation of the bacterial cells; aggregation was not detected when clusterin was added to B. subtilis BR151 or E. coli JM109 cultures. These results suggest that at least some S. aureus strains possess specific proteinaceous receptors for clusterin. Such receptors may be an important new bacterial virulence determinant for S. aureus, as clusterin has been proposed to have a role in the regulation of complement activity.