The exotoxin produced by Vibrio cholerae is rapidly and firmly bound to the outer membrane of mammalian cells. With simple in vitro and in vivo methods and very pure gangliosides and allied glycolipids we have demonstrated that the monosialosylganglioside GM1 is the natural receptor for the cholera toxin. This ganglioside binds the toxin with a high affinity and inactivates it. The inactive derivative, choleragenoid toxoid has the same affinity to GM1 as the toxin. Ganglioside GM1 was isolated from the small intestinal mucosa of man, pig and ox in amounts of 0.1, 2.0 and 43 nmoles per g mucosa, respectively. These very large differences in the ability of the mucosal cells to bind cholera toxin. Exogenous GM1 was incorporated in vitro and in vivo in intestinal mucosal cells. The incorporation of GM1 increased the number of toxin-binding sites and increased the secretion of fluid in the gut. Vibrio cholerae sialidase did not hydrolyse the di- and trisialogangliosides of intact mucosal cells to the parent GM1-ganglioside, neither did it increase the number of cholera toxin-binding sites.