The rate of shigella toxin entry into the cytosol of HeLa S3 cells was estimated from the toxin-induced reduction in protein synthesis. Whereas high toxin concentrations strongly reduced protein synthesis within 30 min, lower concentrations required longer times. The major part of the cell-bound toxin entered only after several hours. Toxin entered cells after incubation at 25 degrees C but not at 20 degrees C, although toxin binding was the same at the two temperatures. Increasing the KCl concentration to 0.2 M protected against toxin. The toxin entry was strongly reduced when the level of ATP in the cells was reduced by incubation with metabolic inhibitors. Lysosomotrophic agents such as NH4Cl and chloroquine had little or no protective effect, but the protonophores carbonyl cyanide p-trifluoromethoxyphenylhydrazone and carbonyl cyanide m-chlorophenylhydrazone and the ionophore monensin protected cells against the toxin. Cells were also protected when the pH was reduced to 6.4. The entry of shigella toxin is discussed in relation to that of other protein toxins with intracellular sites of action.