Two immunologically distinct verotoxins purified from Escherichia coli C600, lysogenized with distinct temperate phages from E. coli strain 933 of serotype O157:H7, were compared by SDS-PAGE and different biological assays. The two toxins termed verotoxin 1 (VT1) and verotoxin 2 (VT2) differing in molecular weight exhibited similar biological activities. Both preparations were toxic for HeLa cells and lethal for mice. Epidemiological evidence of verotoxinogenesis in some cases of hemolytic-uremic syndrome (HUS) and the recent observations of inadequate prostacyclin production by endothelial cells associated with HUS prompted us to study the effect of purified verotoxins on prostacyclin synthesis in rat aortic tissue. Our results demonstrate a significant reduction of prostacyclin by both toxins at picomolar levels. The suppression of prostacyclin release by a lower concentration of VT2 as compared with VT1 reflects the relative potencies of these toxins in HeLa cell toxicity and mouse lethality. The results suggest an effect of verotoxins on endothelial cells and support the concept of these toxins as virulence factors in E. coli.