Bacterial penetration into epithelial cells, scraped from the palatine tonsils of 14 patients (10 males, four females; median age 16 years) with current infectious mononucleosis and concomitant membranous tonsillitis, was studied using the transmission electron microscopic (TEM) technique. Bacteria were seen to adhere to and penetrate the epithelial cells, some of which were completely filled with bacteria. This finding suggests intracellular proliferation of bacteria. Epstein – Barr virus, the causative agent of infectious mononucleosis, especially when associated with growth of β-haemolytic streptococci on the palatine tonsils, induces bacterial penetration into tonsillar tissue, that in turn might be a causative mechanism in the development of peritonsillar abscess.