Ultraviolet difference absorption spectra of cholera toxin and its B protomer produced by the oligosaccharide moiety of the monosialoganglioside GM1 were measured as a function of the oligosaccharide concentration. In the presence of oligosaccharide, the spectrum is characterized by three peaks at 282, 288, and 292 nm. A linear increase in difference absorption was observed at these wavelengths vs. oligosaccharide concentration; a saturation effect occurred when the molar ratio of oligosaccharide to cholera toxin was higher than 5. The features of the spectra indicated that the binding with the oligosaccharide affected the environment of tryptophan and tyrosine residues of protomer B. In good agreement with the above results, circular dichroic spectra indicated also a local effect of the binding, mostly restricted to protomer B, while the residues of protomer A remained largely unperturbed. Difference absorption spectra were also measured for cholera toxin in the presence of ganglioside and detergent micelles. The employed gangliosides GD1a and GT1b, unable to bind cholera toxin, interact with the protein by way of contaminating traces of GM1. The preparations of GD1a and GT1b contained 0.8-1.0% (w/w) and 0.4-0.5% (w/w) of GM1, respectively. The results obtained with ganglioside GD1a and GT1b in contrast with the observations made with the oligosaccharide of GM1 indicated a major conformational change of the toxin structure. Upon comparison of the conformational change induced by ganglioside micelles with that induced by sodium dodecyl sulfate it may be suggested that the ganglioside micelle, behaving as a detergent, alters the structure of the toxin such as to induce the penetration of protomer A into the lipid milieu.(ABSTRACT TRUNCATED AT 250 WORDS)