Antigenic surface properties of Staphylococcus aureus strains grown in milk whey were compared with TSB-grown bacteria using immuno-gold electron microscopy. It is shown that colloidal gold (CG) particles coated with polyclonal antibody raised against Staphylococcus aureus surface antigen expressed in vivo bound to the surface of S. aureus strain F1440 grown in milk whey, but not to homologous bacteria grown in TSB. S. aureus strains grown in milk whey agglutinated in the presence of the polyclonal antibody, whereas the corresponding bacteria grown in TSB did not agglutinate. Immuno-gold particles did not bind to milk whey-grown bacteria treated with periodate. Periodate-treated milk whey-grown bacteria did not agglutinate in the presence of the polyclonal antibody, whereas periodate treatment had no effect on TSB-grown bacteria.