FITC-labeled cholera toxin subunit B (CTB) stained the surfaces of cells of mucous acini in the submandibular gland. CTB, also called choleragenoid, binds to the GM1 glycolipid in the cell membrane. The binding in most acini was inhibited by periodic acid oxidation of the sections, while some acini remained unaffected even after increased oxidation. Staining with the subunit was also reduced significantly by adding galactose to the incubation medium. Binding of CTB to cell surfaces apparently requires intact sialic groups on most, but not all, cell surfaces. Oxidation of the sialic acid residues may influence the structure of the sialylated GM1 molecules on the cell surface in different ways. It is possible that both the sialic acid residue and the terminal galactose are oxidized. Alternatively, the sialic acid may be resistant to acid hydrolysis in gangliosides in which the sialic acid is attached to the internal galactose residue linked to GalNAc, as in the GM1 glycolipid. Inhibition of the GM1 receptor binding to cholera toxin has potential for protection of humans against cholera. Galactose and agents that modify sialic acid inhibit the accessibility of the toxin to the GM1 carbohydrate receptor. Human milk contains high levels of sialic acid glycoconjugates that may provide defense mechanisms.