Shiga and Shiga-like toxins are potent bacterial cytotoxins composed of six proteins: one A-subunit that possesses a toxic N-glycosidase activity and a pentamer of identical B-subunits that anchors the toxin to glycolipids present on mammalian cells. Following their endocytosis through coated pits, a segment of the A-subunit noncovalently associated with the B oligomer is translocated to the cytoplasm where it enzymatically inactivates the protein synthesis machinery. The fluorescence intensity of the single tryptophan residue in the B-subunit is perturbed by pH conditions typically observed in an endosomal compartment, with a sharp reversible transition in fluorescence intensity occurring at pH 4.5. The secondary structure of the pentamer as monitored by circular dichroism is altered by pH conditions lower than 4.5 and greater than 10. However, the conformational change observed under acidic conditions as low as pH 2 does not parallel a loss of receptor binding potential and is reversible, suggesting that the structure of the B-subunit undergoes a second conformational change between pH 4.5 and 2 without a loss of tertiary or quaternary structure. The B-subunit may thus play a role in the translocation of the A chain to the cytoplasm, an event potentially mediated by a conformational change in its structure at pH levels occurring in the endosomal or lysosomal compartments.