Experimental infections of mice with strains of Listeria spp. isolated from contaminated food sources allowed discrimination of strains into those either exhibiting high, attenuated or low virulence. Compared to the highly virulent L. monocytogenes strain EGD, an attenuated strain such as L99 persisted for shorter times (5 versus 10 days) in the infected host. Using a tissue culture cell model of infection, we found that, although strain L99 was capable of accumulating actin like its virulent counterpart following invasion, it was unable to generate the polarized actin tails required for intracellular and cell-to-cell movement. Immunoblot analysis using specific antiserum to the ActA polypeptide, a molecule that is necessary for movement of the bacterium within the eucaryotic cell, indicated that a slightly truncated form of this polypeptide was produced in the L99 strain. Despite its reduced virulence, the attenuated strain L99 was just as effective in generating protection in immune mice as the highly virulent strains, albeit with a 1000-fold higher infective dose. Based on the results obtained from this study, we suggest that one of the mechanisms accounting for widespread resistance in humans to infection by Listeria may be due to asymptomatic infections by naturally occurring strains attenuated for virulence.