Epithelial cells and bacteria were sampled from the tonsillar surfaces of seven patients (six males, one female; median age 16 years, range 10 to 21 years) suffering from acute infectious mononucleosis with concomitant pharyngotonsillitis. By using gold-labelled antiserum to human IgG and secretory IgA (sIgA), micro-organisms on the tonsillar surfaces coated with these immunoglobulins could be identified by tracing the gold particles in the transmission electron microscope. The patients harboured significantly fewer bacteria coated with immunoglobulins than did healthy controls. More bacteria were coated with IgG immunoglobulins than with sIgA. Reduced immunoglobulin-coating of the bacteria on the tonsillar surfaces during infectious mononucleosis can explain their tendency to immense local colonization and proneness to penetrate into the epithelial cells.