Clostridium difficile causes pseudomembranous colitis in humans. The enterotoxin (i.e., toxin A) from this organism is believed to be responsible for the initial intestinal pathology associated with this disease. Previous work shows that this toxin binds to carbohydrates that contain Gal alpha 1-3Gal beta 1-4GlcNAc. However, this carbohydrate is not present on normal human cells. Therefore, this study was undertaken to identify potential receptors for toxin A that do exist on human intestinal epithelium. Using enzyme-linked immunosorbent assay, affinity chromatography, and altered migration in an electric field, we assayed the binding of toxin A to purified carbohydrates and glycoproteins. We found that toxin A bound to the carbohydrate antigens designated I, X, and Y. Each of these carbohydrates exist on the intestinal epithelium of humans.