Samples from the fur covering the palatine tonsils of 10 patients with infectious mononucleosis (IM) were subjected to fluorescence microscopy after staining with acridine orange. Standard aerobic cultures from swabs were set up simultaneously. The fur constituted a mass of variously sized cocci and rods intermingled with granulocytes and cellular detritus. Five patients had growth of beta-haemolytic Streptococci on the tonsils. Bacteria in abundance were found attached to the tonsillar epithelial cells. The B-lymphotrophic Epstein-Barr virus, responsible for the IM infection, causes an immense increase in the bacterial colonization of the tonsillar surfaces, probably due to an increased attachment of bacteria to the tonsillar epithelial cells.