Escherichia coli heat stable enterotoxin (STa) and the newly identified endogenous ligand guanylin bind to an intestinal receptor and activate membrane bound guanylate cyclase. We compared STa binding and affinity crosslinking of STa receptors in human small intestine to those in the Caco-2 human colon carcinoma cell line. STa had similar kinetics of binding in human intestinal and Caco-2 brush border membranes. In both human intestine and Caco-2 brush border membranes, multiple specifically radiolabeled bands, including a 140-165 kDa band, were identified by affinity crosslinking. However, in human intestine the most prominent autoradiographic species was a 60 kDa band. A 60 kDa protein was also specifically immunoprecipitated from solubilized human brush border membranes using antisera raised against a cloned STa receptor fusion protein. Our observations of multiple crosslinked proteins in human intestine and Caco-2 cells could be explained by the existence of several members of a family of STa receptors and/or the existence of smaller STa binding proteins generated by the protease cleavage of a larger complete STa receptor.