Salmonella enterica serotype Enteritidis (S. enteritidis) is a major food-borne pathogen, and its incidence among all Salmonella serotypes has increased dramatically in the last two decades. To study the virulence characteristics of clinical isolates of S. enteritidis, we determined the 50% lethal doses (LD₅₀) in mice of isolates of two major phage types (4 and 8). Isolates of both phage types showed a wide range of LD₅₀ after oral inoculation, varying from under 10² organisms to over 10⁸ organisms. No significant difference in LD₅₀ was observed between the phage types. These observations indicated that clinical isolates ofS. enteritidis are highly heterogeneous in their ability to cause death in mice. We compared the LD₅₀s of these isolates to the results observed from in vitro pathogenicity assays. We also analyzed these isolates for recognized Salmonellavirulence loci (spv, sodCI, sopE, and sef). The in vitro phenotypes of the isolates showed no obvious correlation with their LD₅₀ in any given assay, and the virulence genes tested were present in all isolates. However, the isolate with the lowest LD₅₀ (isolate 97A 2472) was resistant to acidified sodium nitrite (ASN). Moreover, the most acid-susceptible, macrophage-susceptible, and ASN-susceptible isolates were attenuated for virulence in mice. These results, based on extensive analysis of clinical isolates of S. enteritidis, demonstrate the complex nature of Salmonella pathogenesis in mice. Our results also indicate the limitation of in vitro assays in predicting in vivo virulence.