In earlier studies using a streptomycin-treated mouse model of infection caused by enterohemorrhagic Escherichia coli (EHEC), animals fed Shiga-like toxin type II (SLT-II)-producing strains developed acute renal cortical necrosis and died, while mice fed Shiga-like toxin type I (SLT-I)-producing clones did not die (E. A. Wadolkowski, L. M. Sung, J. A. Burris, J. E. Samuel, and A. D. O'Brien, Infect. Immun. 58:3959-3965, 1990). To examine the bases for the differences we noted between the two toxins in the murine infection model, we injected mice with purified toxins and carried out histopathological examinations. Despite the genetic and structural similarities between the two toxins, SLT-II had a 50% lethal dose (LD50) which was approximately 400 times lower than that of SLT-I when injected intravenously or intraperitoneally into mice. Histopathologic examination of toxin-injected mice revealed that detectable damage was limited to renal cortical tubule epithelial cells. Passive administration of anti-SLT-II antibodies protected mice from SLT-II-mediated kidney damage and death. Immunofluorescence staining of normal murine kidney sections incubated with purified SLT-I or SLT-II demonstrated that both toxins bound to cortical tubule and medullary duct epithelial cells. Compared with SLT-I, SLT-II was more heat and pH stable, suggesting that SLT-II is a relatively more stable macromolecule. Although both toxins bound to globotriaosylceramide, SLT-I bound with a higher affinity in a solid-phase binding assay. Differences in enzymatic activity between the two toxins were not detected. These data suggest that structural/functional differences between the two toxins, possibly involving holotoxin stability and/or receptor affinity, may contribute to the differential LD50s in mice.