C. difficile, produces 2 potent cytopathic enterotoxins, A and B; their mechanism of action is poorly understood. We studied effects of enterotoxin A (EA) on cultured Chinese Hamster Ovary (CHO) cells employing light microscopy and TEM. Cells grown to 75% confluency were exposed to EA (2.5 μg/ml) at 37°C. Within 1-3 hrs cytoplasm became more retractile and contracted with thin protrusions from the cell (Fig. B). The cells were washed after 2 or 3 hr and incubation continued for 48 hrs. If EA was removed within 2 hrs cells returned to normal in 24-48 hr; if EA was removed after 3 hrs it did not affect rounding of the cells which remained firmly attached and not permeable to erythrosin B implying that they were viable.TEM employing monoclonal Abs and colloidal gold-protein A conjugate showed that at 4°C EA was randomly distributed on plasma membrane (PM). At 37°C EA was located in pinocytotic invaginations and in coated pits (Figs. E-H) suggesting EA internalization by both the bulk and receptor-mediated endocytosis.