One half million patients suffer from colorectal cancer in industrialized nations, yet this disease exhibits a low incidence in under-developed countries. This geographic imbalance suggests an environmental contribution to the resistance of endemic populations to intestinal neoplasia. A common epidemiological characteristic of these colon cancer-spared regions is the prevalence of enterotoxigenic bacteria associated with diarrheal disease. Here, a bacterial heat-stable enterotoxin was demonstrated to suppress colon cancer cell proliferation by a guanylyl cyclase C-mediated signaling cascade. The heat-stable enterotoxin suppressed proliferation by increasing intracellular cGMP, an effect mimicked by the cell-permeant analog 8-br-cGMP. The antiproliferative effects of the enterotoxin and 8-br-cGMP were reversed by L-cis-diltiazem, a cyclic nucleotide-gated channel inhibitor, as well as by removal of extracellular Ca(2+), or chelation of intracellular Ca(2+). In fact, both the enterotoxin and 8-br-cGMP induced an L-cis-diltiazem-sensitive conductance, promoting Ca(2+) influx and inhibition of DNA synthesis in colon cancer cells. Induction of this previously unrecognized antiproliferative signaling pathway by bacterial enterotoxin could contribute to the resistance of endemic populations to intestinal neoplasia, and offers a paradigm for targeted prevention and therapy of primary and metastatic colorectal cancer.