Swiss mice were infected with two Listeria strains: L. monocytogenes strain 10401, serovar 4b, and L. innocua strain 390, serovar 6a. Bacteria were inoculated by intravenous, subcutaneous or oral routes, and then enumerated in the spleen. The splenic infection was studied comparatively for these three inoculation routes with both strains. Strain 390 caused a splenic colonization only after intravenous inoculation. For the 10401 strain, the peak of infection appeared on the 3rd day after inoculation; the intravenous route was the most efficient to kill mice, the subcutaneous one the most efficient to obtain a reproductible sublethal infection; the oral route infected regularly only with doses higher than 10(7) bacteria. A splenomegaly appeared only in mice infected with strain 10401. Estimation of Listeria strain pathogenicity depended more on the measure of bacterial count in the spleen on the 3rd day of infection than on LD50. To be able to compare quantitatively the pathogenicity of L. monocytogenes and L. innocua, it seemed impossible to use only one dose of bacteria and to inoculate through one route.