We show here that the susceptibility of endothelial cells to Shiga toxin (Stx)s differs remarkably depending on their cellular origins. The concentration of Stx-1 required to reduce cell viability by 50% as measured by MTT assay was 30 and 300 fM for neonatal and adult human microvascular endothelial cells (HMVEC), respectively, and 30 pM for human coronary artery endothelial cells (HCAEC). Human umbilical venous endothelial cells (HUVEC) and bovine aortic endothelial cells (BAEC) showed no sensitivities to Stx-1. Surprisingly, Stx-2 was approximately 10–100 times more toxic to HMVEC than Stx-1. Moreover sodium butyrate sensitized HMVEC by 100-fold to the cytotoxic activity of Stxs. These results were found to reflect the amount of Gb3/CD77 on the cell surface on a per cell basis using flow cytometrical analysis. The high sensitivity of HMVEC to Stxs suggests their involvement in the pathogenesis of organ failure induced by Stx-producingEscherichia coli.