The carboxyl group of the natural cholera toxin receptor, the ganglioside GM1, Gal beta 1-3GalNAc beta 1-4(NeuAc alpha 2-3)Gal beta 1-4Glc beta 1-Cer, has been converted to a number of C(1)-amides of NeuAc. The binding of cholera toxin B-subunit to these derivatives was monitored by exposing the modified glycolipids, on solid phases, to radiolabeled toxin. Binding was obtained, although substantially reduced, with the amide and to a lesser extent with the benzylamide and also the C(1)-alcohol. In the assay system used, the methyl-, ethyl-, or propylamides did not bind. It was concluded that the hydrogen bonding capacity of a carboxyl or amide group is needed for strong binding. This is in agreement with the recently published crystal structure of the B-subunit in complex with the GM1 pentasaccharide.