A simple assay for the heat-stable enterotoxin (ST) of Escherichia coli was developed on the basis of ST activation of guanylate cyclase in membranes from the intestinal mucosa of mice. ST activated guanylate cyclase in mucosal membranes in a linear fashion over a 50-fold range of toxin concentrations with Mg++-guanosine 5'-triphosphate as substrate. Activation of guanylate cyclase was detectable at concentrations of ST that were five- to 10-fold lower than those resulting in increases in the ratio of gut weight to carcass weight of mice. This assay was used to quantify ST in crude and purified samples from culture filtrates of wild-type strains and recombinant strains of E coli containing the gene for ST. Activation of guanylate cyclase was specific for ST; purified cholera toxin and E coli heat-labile enterotoxin did not activate guanylate cyclase. Thus, this assay for ST is sensitive, specific, and will facilitate rapid analysis of samples for quantification of ST.