The glycolipid-binding cytotoxin produced by Shigella dysenteriae 1, Shiga toxin, binds to MDCK cells (strain 1) only after treatment with short-chain fatty acids like butyric acid or with the tumor promoter 12-O-tetradecanoylphorbol 13-acetate. The induced binding sites were found to be functional with respect to endocytosis and translocation of toxin to the cytosol. Glycolipids that bind Shiga toxin appeared at both the apical and the basolateral surface of polarized MDCK cells grown on filters, and Shiga toxin was found to be endocytosed from both sides of the cells. This was demonstrated by EM of cells incubated with Shiga-HRP and by subcellular fractionation of cells incubated with 125I-labeled Shiga toxin. The data indicated that toxin molecules are endocytosed from coated pits, and that some internalized Shiga toxin is transported to the Golgi apparatus. Fractionation of polarized cells incubated with 125I-Shiga toxin showed that the transport of toxin to the Golgi apparatus was equally efficient from both poles of the cells. After 1-h incubation at 37 degrees C approximately 10% of the internalized toxin was found in the Golgi fractions. The results thus suggest that glycolipids can be efficiently transported to the Golgi apparatus from both sides of polarized MDCK cell monolayers.