The potential of a new optical biosensor, the resonant mirror, for detecting whole cells is demonstrated. Staphylococcus aureus (Cowan-1) cells, which express protein-A at their surface, were detected by binding to human immunoglobulin G (IgG) immobilized on an aminosilane-derivatized sensor surface at concentrations in the range 8 x 10(6)-8 x 10(7) cells/mL. A control S. aureus strain (Wood-46), which does not express protein-A, gave no significant response. Immobilization of the capture ligand on aminosilane surfaces with and without a hydrogel coating of carboxymethyl-dextran was compared. The greatest binding response was observed with non-dextran-coated surfaces. The sensitivity of the technique was increased a 1000-fold by using a human IgG-colloidal gold conjugate (30 nm) in a sandwich assay format. S. aureus (Cowan-1) cells were detected in spiked milk samples at cell concentrations from 4 x 10(3)-1.6 x 10(6) cells/mL using the sandwich assay.