Bacterial samples were obtained from the tonsillar surfaces of seven patients (four males, three females; median age 18 years, range 15 to 21 years) suffering from acute infectious mononucleosis with concomitant pharyngotonsillitis, and from five healthy controls. By using gold-labelled antiserum to human lysozyme and lactoferrin, micro-organisms on the tonsillar surfaces coated with these antibacterial substances could be identified by tracing the gold particles in the transmission electron microscope. In healthy individuals, most of the bacteria were coated with lysozyme and significantly more bacteria were coated with lysozyme than with lactoferrin (p < 0.01). In patients there was a non-significant reduction in lyoszyme-coating of the bacteria, whereas lactoferrin-coating was significantly increased (p < 0.01). Changes in the lysozyme and/or lactoferrin coating of the tonsillar surface bacteria on the palatine tonsils during infectious mononucleosis cannot explain the tendency to immense local bacterial colonization with commensals and proneness to bacterial penetration into the epithelial cells.