The enterotoxin of Vibrio cholerae, the heat-labile enterotoxins of Escherichia coli, and adrenocorticotropin (ACTH) induce morphogenic and steroidogenic changes in Yl adrenal tumor cells in monolayer tissue culture. A clonal mutant cell line (OS3) of these Yl cells that does not respond to ACTH but is stimulated by adenosine-3′,5′-monophosphate (cyclic AMP) to increase its production of Δ4,3-ketosteroids is similarly stimulated by V. cholerae and heat-labile E. coli enterotoxins. These results indicate either that different receptor sites exist for ACTH and the two toxins or that different adenyl cyclases are being activated by the pituitary hormone and the enterotoxins. OS3 cells in tissue culture, as well as the Yl adrenal cell line, should serve as useful model systems with which differences in receptor sites or molecular mechanisms of action between the V. cholerae and heat-labile E. coli enterotoxins and any other activators of the adenyl cyclasecyclic AMP systems can be readily investigated.