The morphologic change in Chinese hamster ovary (CHO) cells that is associated with increased intracellular concentrations of adenosine 3':5'-cyclic phosphate (cyclic AMP) provided a highly sensitive assay for the heat-liable enterotoxins of Vibrio cholerae and Escherichia coli. The similarity between the responses of CHO cells and those of intestinal mucosa to enterotoxins was demonstrated by the inhibition of these effects by the ganglioside galactosyl-N-acetylgalactosaminyl [sialosyl] lactosyl ceramide (GM1), as well as by specific antisera. The antisera against either enterotoxin were more effective in inhibiting the effects of homologous toxin than those of heterologous toxin, but cross-neutralization also occurred in both cases. When choleragenoid, the natural toxoid of cholera toxin, was incubated with the CHO cells, the subsequent effects of the toxin on the morphology of the cells were prevented as were the effects of the toxin on intestinal secretion. However, choleragenoid did not prevent the effects of E. coli toxin. The similarity between the action of enterotoxins on CHO cell morphology and on secretion by the small intestine suggests that there are similar binding sites in these tissues. The potential usefulness of the CHO cell assay in quantitative measurement of antitoxic immunity is thus demonstrated.