The intracellular parasite Listeria monocytogenes is able to induce its internalization by cultured mammalian cells that are not normally phagocytic. This process requires the expression of the chromosomal locus inlAB. We studied the virulence of an inlAB mutant and of its parent in murine listeriosis. Irrespective of the route of inoculation, the inlAB mutant was severely attenuated for growth in the liver. The livers of mice inoculated with the inlAB mutant displayed much smaller infectious foci than the parent as early as 24 h after infection. Electron microscopy showed that these foci consisted of a few inflammatory cells, with few bacteria; bacteria were rarely found within hepatocytes. In contrast, foci in livers of mice inoculated with the parent consisted of islets of heavily infected hepatocytes that were infiltrated by numerous neutrophils; bacteria seemed intact within hepatocytes and damaged within neutrophils. A direct role of inlAB for the entry of L. monocytogenes into hepatocytes was confirmed in a cell infection system using the murine embryonic hepatocyte cell line TIB73. The inlAB mutant was approximately 20-fold less invasive in trans. The "invasion locus" inlAB contributes to protect L. monocytogenes from the host's innate defense mechanisms by promoting its entry into hepatocytes.