Sheep immunized with live or inactivated vaccine were challenged with a virulent strain of Listeria monocytogenes. Clinical manifestations, penetration of listeriae into the mucous membranes and persistence in organs were noted. Bacteriaemia caused by intravenous injection of high doses of virulent strain disappears in 3 days. Readily after inoculation, listeriae penetrate into the gastrointestinal tract where they persist for 8 days. Listeriae appear from the 3rd day in the conjunctiva and nasal mucous membranes with clinical symptoms of conjunctivitis and rhinitis whence they are eliminated in 8 to 14 days. The brain, liver and spleen of animals that died in 5 to 7 days were loaded with listeriae. Organs from immune animals sacrificed 14 days after challenge were negative by cultivation even when listeriomas were in the liver and spleen. Rapid penetration and persistence of listeriae are an expression of their increased affinity to epithelia of systems which are not only the portal of their entry but even the site of their secondary propagation and thus a source of infection.