Shiga toxin is enterotoxic in rabbit small bowel and binds to the microvillus membrane (MVM). The toxin exhibits specificity for glycolipids possessing a terminal gal-alpha 1----4gal disaccharide, including the neutral glycolipid Gb3 in MVM. Gb3, which is developmentally regulated in the rabbit small bowel, is present in very low concentration until the animals reach day 16 of life. In older animals an increase in Gb3 content is paralleled by an increase in the ability of MVM to bind toxin, which also correlates with the fluid secretion response. Shiga toxin selectively binds to villus cells, which contain Gb3, and not to crypt cells, which do not express Gb3. Targeting of the villus cell by the toxin is consistent with physiologic studies that demonstrate inhibition of villus cell Na+ absorptive pathways, with no effect on crypt cell Cl- secretory mechanisms. These effects are sufficient to account for the enterotoxicity of Shiga toxin in the rabbit model.