Listeriosis is a severe disease (meningitis, abortion, septicemia) with a high case-fatality rate (20-30% of cases) which is mainly observed in industrialized countries. Host susceptibility play a major role: a large majority of patients have an underlying condition which predisposes to infection by interfering with T-cell mediated immunity [extreme ages (neonates and the elderly), pregnancy, cancer and immunosuppressive therapy, AIDS]. Epidemiological investigations of outbreaks and sporadic cases have associated a great variety of food to the disease. Data accumulated during the past ten years indicate that highest risk foods are often ready to eat, stored at refrigeration temperature for a long period, enabling Listeria to grow, contaminated with a high concentration of L. monocytogenes( = or \gr 100/g or ml) and moreover by a strain serovar 4b.